Grungy academics resent the elitist notion of genius, but the best writers on art struggle to explain a miracle  the capacity of human beings to perform the godlike task of creating new worlds, making familiar reality freshly radiant and transforming sight into vision.

Sometimes, however, the genius is a bewilderingly ordinary man. Sheila Hale's task in Titian: His Life (Harper Press £30) is complicated by the fact that although the greatest of painters had a long life, during which he was feted as a celebrity and deferred to by the potentates he painted, he left few personal traces. This Old Master didn't disport himself with mistresses or, like Leonardo and Michelangelo, with ephebic young men; most of his surviving correspondence chases unpaid bills. Hale calls Titian's portraiture "Shakespearean", but like Shakespeare the connoisseur of human individuality remains somehow anonymous, nondescript. The baffled biographer compensates by evoking the "tremendous times" in which Titian lived and the imperial excesses of Venice, a city so luxuriously decadent that nuns, in those benighted days before the invention of the vibrator, consoled themselves with dildos crafted from Murano glass.

Michael Hirst's Michelangelo: The Achievement of Fame, 1475-1534 (Yale £30) is less of a rowdy epic. Hirst's account of the Sistine Chapel fusses over pigments and scaffolding, and in the process misses the bigger picture that extends across the ceiling. Half the volume is footnotes; more agony and ecstasy would have been appreciated. Ross King has problems of his own in Leonardo and the Last Supper (Bloomsbury £20): how can we comprehend a mural that began to curdle and fester almost as soon as it was painted and is now a blotchy smear? But King draws attention to overlooked iconographic details such as the supper menu, and he profits from the mural's misfortunes in a brilliant history of its afterlife as restorers perform cosmetic surgery on it.

Hero worship, so critically unfashionable, triumphantly returns in Alex Danchev's Cézanne: A Life (Profile £30), which is as much a religious pilgrimage and a philosophical flight of fancy as a conventional biography. Hockney once said that before Cézanne no one looked at the world with both eyes; Danchev explains how he broke "the skin of things" and exposed their structure.

Danchev can be loftily obscure, but better this than the bluff populism of Will Gompertz in What Are You Looking At? 150 Years of Modern Art in the Blink of an Eye (Viking £20), where Cézanne is described as "a DIY dad taking on a minor household chore only to find himself knee-deep in unforeseen complications". Gompertz began as a Tate curator, retrained as a standup comedian, and now presents arts programmes on the dumbed-down BBC; given this career path, it's unsurprising to find him calling Le Douanier Rousseau "the Susan Boyle of his day". Television has afflicted Gompertz with the agitated blink rate of a fibber: alas, those 150 modernist years can't be compressed into a few seconds of screen time and a string of gawky jokes.

Modern artists speak about one another in In My View, edited by Simon Grant (Thames & Hudson £19.95), a superb collection of testimonials recalling childhood epiphanies in museums or libraries, moments that now seem like the start of a new artistic vocation. Most of the encounters are brilliantly unexpected. Miroslaw Balka prefers his grubby, grainy reproduction of Michelangelo's Pietà to the original, which is "too clean and white"; Zhang Huan recreates The Last Supper by smearing incense ash on linen, replacing the Christian story with a Buddhist homage to "the souls of mankind". Chuck Close, whose own work dissects visible reality, admires Vermeer because his images can't be deconstructed: the pigment is "blown on to the canvas by a divine breath of air". Bill Viola is equally reverent in his essay on Bellini. Unashamed about the tears he shed when he discovered The Dead Christ Supported by Angels in the National Gallery, Viola even writes a letter to his long-dead predecessor asking: "Now that you are gone from this earth, what images are you seeing now?"

Finally, two sumptuous studies of the body as a work of art when clothed, or as a subject for art when unclothed  Nathalie Herschdorfer's Coming into Fashion (Thames & Hudson £42) and Frances Borzello's The Naked Nude (Thames & Hudson £28). Herschdorfer raids the Condé Nast archives for her account of the relations between models and the designers and photographers who tortured them into beauty, while Borzello brilliantly updates the evasively genteel distinction between naked and nude made by Kenneth Clark in 1956, exposing the new qualms of conscience that have emerged in our own franker but more scrupulous era. Borzello's illustrations are titillating, embarrassing and sometimes outright disgusting: here we find art holding a mirror up to our troublesome human nature and showing us who we are, genitals and all.

So equally matched were the four greatest painters of the High Renaissance that neither their contemporaries nor posterity have been able to separate them for long. Each has had his adherents and each his periods of supremacy but in the end they are best defined by their distinctive gifts: if Leonardo's work represents the age's spirit of enquiry, then Michelangelo's personifies its terribilità  its frightening power, Raphael's its physical perfection and Titian's its humanity.

Of the four it is Titian who has always stood slightly apart. As a Venetian he was not only geographically separate but had different artistic roots and a different sensibility from the others, who shared a Florentine training. In the contemporary debate about the primacy of disegno (design) or colorito (colouring) it is three against one, with Titian the sole exemplar of colour over line. Yet while Michelangelo and Raphael were tagged "divine" and Leonardo universally held as sui generis it was Titian who the great art historian Bernard Berenson claimed had portrayed "nearly all of the Renaissance that could be expressed in painting".

Although she does not explicitly say so, Berenson's opinion is shared by Sheila Hale, the author of a huge and exceptional new study of the painter, Titian: His Life (HarperPress, £30)  remarkably this is the first full biography since 1877. It also finds sympathy with the panjandrums of the National Gallery and the Royal Ballet, whose institutions are collaborating, as part of the London 2012 festival, with Metamorphosis, which involves specially commissioned pictures and ballets inspired by Titian's paintings after Ovid's stories. If the turn of the year belonged to Leonardo with the unprecedented exhibition at the National Gallery this part of 2012 is Titian's.

Lucian Freud described the three pictures at the heart of this painting/dance cooperation  Diana and Actaeon, Diana and Callisto and The Death of Actaeon  as "quite simply the most beautiful pictures in the world". Titian's ability to beguile and inspire comes not just from his mythological works though: his religious paintings were celebrated for their extraordinary emotive force (when Titian's patron, the Holy Roman Emperor Charles V, died in 1558 he was looking at the painter's Adoration of the Trinity), while WB Yeats found in his portraits "the personality of the whole man, blood, imagination, intellect running together".

Titian's fame in his own lifetime meant that there are several contemporary accounts of his life and works  not least by Vasari, the father of art history, in his second edition of The Lives of the Artists. Hale's book, however, traces in unprecedented detail not just Titian's long career (he lived apocryphally to 100, although 86 is now his commonly accepted age) but the Venice in which he spent almost all his life  at once intellectually avant-garde, beautiful and bloody. Because Titian was the only artist to paint a pope (Paul III), an emperor (Charles V), a sultan (Suleiman the Magnificent) and a king (Philip II/Francis I), Hale's is also a book about great men, among whom the artist could number himself.

The man who emerges does not easily fit the template of genius. Written accounts suggest that throughout his life, art was primarily a business. The bulk of his surviving correspondence deals with money matters, not least the endless trouble he had in getting paid for his paintings. He left no comment about art's higher calling, the creative process or about religion or politics. His health was good, he stayed within easy reach of Venice and travelled as far as Rome only once and outside the Italian peninsular just twice (across the Alps to Augsburg). His was a life largely without incident. Leonardo and Michelangelo are thickly encrusted with stories but Titian is defined by a single well-known anecdote that suggests his stature: when he was painting Charles V he dropped his brush which the emperor stooped to pick up; "Titian protested, saying 'Sire, I am not worthy of such a servant.' To which the emperor replied: 'Titian is worthy to be served by Caesar.'"

Titian's route to such eminence was, like that of his peers, smooth if not effortless. He was born around 1490 to a noble but not patrician family in Pieve de Cadore, a mountain village 110 kilometres north of Venice to which he remained faithful all his life. He was sent to Venice as a half-educated boy to learn painting. La Serenissima was a place of spectacular wealth through its trade with the eastern Mediterranean and role as an entrepôt for everything from spices and timber to books and prostitutes. Hale writes that its population of 100,000 was twice the size of today's and it grew to 175,000 during Titian's lifetime. Venice also had a history of religious tolerance and, as a republic, political independence. It kept itself apart from the rest of Italy.

Among his teachers was Giovanni Bellini, the most sublime of Venice's artists, whose pupils also included Giorgione, Sebastiano del Piombo and Palma Vecchio. It was Bellini who first brought the Madonna down from heaven and into the meadows and hills of northern Italy, an example Titian first followed and then adapted for his allegorical pastorals such as The Three Ages of Man (1509) and Sacred and Profane Love (1514). Within a few years Giorgione was dead from the plague, Sebastiano was in Rome and Bellini was looking outmoded, leaving Titian the most celebrated painter in the Veneto.

Titian, as Hale recounts, was singleminded about consolidating this position. He solicited work not just from Venice's civic authorities and religious confraternities but assiduously cultivated the powerful neighbouring dukes of the mainland. His first great benefactor was Alfonso d'Este, the third Duke of Ferrara, a man who traced his ancestry to the Knights of the Round Table and used to walk around Ferrara in the nude. Alfonso in turn lent the painter to his nephew Federico Gonzaga, Marquis of Mantua, a less visceral character but a more discriminating patron.

Titian was invariably slow to deliver the paintings he promised. He would work on several simultaneously and often left them propped against the wall for months or even years before returning to each picture "as if it were a mortal enemy" and starting on it again. Conservators have found dust between the paint layers that testify to his tardiness. Indeed it is thanks to the reports from the duke and marquis's ambassadors in Venice, who were instructed to report on their paintings' progress, that so much is known about Titian's activities. At times they wrote back on an almost weekly basis, retailing among other things Titian's plaintive claim that even if Christ asked him for a painting he would still do Alfonso's first, and the supposition of the duke's man about the painter's ill health: "I suspect that the girls whom he often paints in different poses arouse his appetite, which he then satisfies more than his delicate constitution permits; but he denies it."

This reputation may have had something to do with Titian's friendship with the poet, pornographer and satirist Pietro Aretino  a noted debauchee who was also the painter's greatest proselytiser: "Titian is I and I am Titian," he once wrote; "Messr Titian's paintbrush is my pen." With the architect Jacopo Sansovino they formed what the Doge called Venice's "Triumvirate of Taste". They shared conversation, girls and food. Hale describes an evening when Titian dropped in on Aretino who was roasting some thrushes on a spit. The painter sniffed the birds, looked at the snow outside, ignored an invitation to a dinner that was being held in his honour and settled down with his friend to enjoy the thrushes which were cooked "with a bit of beef, two bay leaves and some pepper".

For all this detail we know frustratingly little about his private life; not even the name of his second wife, the mother of his fourth child, Emilia ("the absolute patroness of my soul"), is recorded. Of his two sons, the eldest, Pomponio, he forced into a career in the church against the young man's will and they didn't speak for the last nine years of Titian's life; the younger, Orazio, became a painter in his own right and his father's studio assistant and man of business.

If the personal relations that shaped his mental world  and hence his paintings  remain opaque his professional life has left a substantial paper-trail, not least regarding the pictures he painted for the Habsburgs from 1532. Titian completed some 150 works for Charles V, his son Philip II of Spain and their courtiers. Through them his work and fame spread from the Low Countries to the Iberian peninsula. Charles knighted him, which gave him the right to appoint notaries and legitimise bastards, while Philip was the recipient of many of his most important pictures, including the Diana paintings of the National Gallery/Royal Ballet collaboration.

The pictures he completed for the Habsburgs included portraits, mythologies and religious scenes and saw him move from his meticulous early style to the bold, expressionistic technique of his later years; pictures that, as Vasari said, were "executed with broad and bold strokes and smudges, so that from nearby nothing can be seen whereas from a distance they appear perfect". Not just the radical appearance of this style but the emotion and complexity of his subject matter  whether it be the portrait of the collector Jacopo Strada, The Flaying of Marsyas or The Crowning with Thorns  put late Titian on a par with Shakespeare, with whom he has much in common, as one of the high-points of western culture.

If Hale's biography is a superb portrait of the artist  an example of measured scholarship, judicious opinion, and telling framing detail  it is the numinous and protean aspects of Titian on display at the National Gallery and Royal Opera House. The three paintings that inspired Metamorphosis are part of the series of pictures known as the "Poesie"  poems  painted for Philip II. Started in 1553 the paintings are studies in erotic passion and its effect on destiny and were designed to arouse the senses and flatter the erudition of his regal patron. The three in the National deal with the implacable goddess Diana; first when she curses the young hunter Actaeon when he stumbles across her bathing naked; second when she turns him into a stag and watches him torn to pieces by his own hounds; and third when she discovers that her handmaiden Callisto has become pregnant by Jupiter.

Three contemporary artists, Conrad Shawcross, Chris Ofili and Mark Wallinger, have reinterpreted the paintings as stage sets for a three-part ballet by seven different choreographers (Wayne McGregor and Christopher Wheeldon among them) with a music score from three modern composers, including Mark-Anthony Turnage. Fourteen poets  including Simon Armitage, Carol Ann Duffy, Seamus Heaney and Christopher Reid  have also written new works inspired by the Titians. It is the sort of cross-artform project that has rarely been seen since the days of Diaghilev. While the performances take place at Covent Garden in mid-July the artists' designs and preparatory works will be on display alongside the Titians in the National.

Titian didn't follow Ovid to the letter  (he couldn't read Latin) but rather in the spirit, and these artists, musicians and dancers are following Titian in the same manner. There is a real sense of appropriateness about the project, too: Titian's paintings were in a sense stage sets, backgrounds to the often theatrical lives of his patrons, and he was also a lover of music, once painting the portrait of a harpsichord-maker in return for an instrument. Above all, the "Poesie", Hale writes, resonate with "the very heartbeat of the antique world" and with this collaboration that rhythm and Titian's own heartbeat still sound strongly.

 Metamorphosis: Titian 2012 is at the National Gallery from 11 July to 23 September (www.nationalgallery.org.uk) and at the Royal Opera House from 14 to 20 July (www.roh.org.uk).