As a young man in Germany in the 1920s Nikolaus Pevsner wrote to himself: "I must become something." His exhortation was not wasted, although what he eventually became was not what he had envisaged. By the time of his death in 1983 aged 81, this Mitteleuropean Jew of Russian descent had become, among other things, an Englishman, a Protestant, a knight of the realm, a staple of the BBC, an academic without portfolio, a publishing phenomenon and, above all, the man who opened the eyes of the English people to their own architecture. Perhaps the greatest of his achievements, though, was to become a noun: when building-fanciers want information about a particular house, church or castle, they don't check a guidebook, they consult a "Pevsner".

The proper name for a "Pevsner" is the Buildings of England series  the 46 county-by-county architectural gazetteers that constitute one of the great enterprises of 20th-century scholarship. Pevsner started them in 1951, and their 60th anniversary has sparked a renewed interest in the man himself. Last year saw the publication of the first volume of Stephen Games's long-gestated biography and, as if to prove the existence of the zeitgeist (a key element in Pevsner's thinking), now comes Susie Harries's dauntingly large life. The very fact that an architectural historian  not the most glamorous of trades  has inspired two such thorough-going biographies is an indication of just how central Pevsner was to postwar British cultural life.

Initially, however, Pevsner wanted simply to be a German. From the age of 13 he kept notebooks in which he confided his most intimate and often embarrassing thoughts. He was embarrassed both by his father being in trade and by his mother's cultural pretensions, but above all by himself. Referring to himself in the third person, Pevsner described his appearance with acute self-loathing: "Ears  dirty, with hair. Hair  long, curly, greasy, with dandruff. Skin  bad  he rarely washes, and then only cursorily."

In his efforts to feel more German he converted to Protestantism, started a body-building programme and developed an enthusiasm for medieval German art. He came to believe deeply in the notion of art as an expression of national character and in the social responsibility of the artist. With the rise of Hitler, these articles of faith placed him uncomfortably close to aspects of Nazi ideology. Although he was never a National Socialist he was politically naive, and his sympathy with some of the party's aims seems to have blinded him to its increasingly pernicious character. Even when he lost his job as an art history lecturer at Göttingen University on account of his Jewishness, he didn't catch on (he was a snobbish antisemite himself). When he came to England in 1933 in search of a new post, he thought the rupture would be temporary and left his family behind in Germany.

He never made it back. Instead he found himself a stranger still, but in a new country. Harries is very good at evoking the atmosphere of the academic milieu into which Pevsner struggled to insert himself. Whereas art history in Germany was a creditable discipline of long standing, in England it was a new subject  class-ridden, based on connoisseurship and, he thought, "at its worst, an activity a bit like stamp collecting". It was this fustian world that Pevsner and other émigrés such as Ernst Gombrich, Rudolf Wittkower and Edgar Wind were to transform.

Pevsner had a hard time fitting in, not least because he was a modernist, something the British found temperamentally uncongenial. For Pevsner, though, it represented an antidote to art for art's sake and he saw it as an appropriate expression of the spirit of the age. Art, he believed, should be functional and of service, and architecture was the most important of the arts because it was the most closely connected with human life. Pevsner was always more of a historian than an aesthete and memorably railed against self-indulgence and "the unbearable oversupply of artists whom nobody wants and whose private feelings are totally uninteresting  let him starve".

Harries tracks in detail Pevsner's efforts to establish himself  hovering at the edges of the Courtauld Institute, a junior post at Edgbaston, writing papers on "The Romance of Rayon" and "The Use of Rubber Furniture". He found success with his primer Pioneers of Modern Design (1936) and more substantially, after a spell as an internee during the first part of the war, with An Outline of European Architecture (1942), which by 1961 had sold 250,000 copies.

These, however, were just precursors to The Buildings of England. He undertook the series for Allen Lane and quickly established an exhausting routine. He would take month-long trips twice a year, covering 2,000 miles at a time in a clapped-out Wolseley, and would visit as many of each county's buildings of note as possible, writing up his notes in dingy B&Bs in the evenings. Turning up unannounced at grand houses, he was frequently refused entry. Once he was let in with the son of the house introducing him to the chatelaine: "They've come to read the meter, Ma." When trying to get in to examine Colney Hatch asylum, with its huge Italianate façade, he told the recalcitrant doorman: "I am Professor Pevsner." "That's a new one," was the response.

Pevsner wrote 32 of the volumes himself and another 10 with collaborators. They are uniformly encyclopedic, opinionated (East Retford is "a singularly unattractive town"; "Cornwall possesses little of the highest aesthetic quality") and written with an evocative vocabulary that can describe buildings or their features as "lanky", "frantic", "victoriously vulgar" or "Grecian gone gaudy". The series lost money but made Pevsner. Harries relates all this with exemplary conscientiousness, and gives full accounts too of the ups and downs of Pevsner's marriage and of his friendships and spats (on their side rather than his) with the likes of John Betjeman and David Watkin.

The author had access to three trunks of unseen private papers, and consequently her book is not far off the size and weight of a breeze block  a material which, as an advocate of functionalism, Pevsner wouldn't have dismissed. Not everything deserves its place but, longueurs aside, this is a very fluent book in which Harries reconstructs not just the man but a time of extraordinary cultural change in which he played such a leading part. As a student Pevsner had written rather piously in his diaries: "It takes earnestness to be a man and diligence to make a genius." He was, shows Harries, both earnest and diligent and made himself a man and, if not quite a genius, someone none the less extraordinary.

"It takes earnestness to make a man and diligence to make a genius," Pevsner noted at 20, and he had plenty of both. He'd started writing historical dramas at seven, and a diary begun in his teens recorded the lifelong anxieties and emotional insecurities that tend to come with precocity of this order.

A Protestant convert, like many of his kind in the early 20th century (his father was a prosperous Russian-Jewish fur trader), he developed an intense patriotism, and in his case quasi-spiritual convictions about the Germanness of German art. For Pevsner, a kind of instinctual, apolitical socialist, national feeling was coupled with a sense of social responsibility, and dislike of the unhealthy values he saw in Weimar Germany.

So it was that in its early days National Socialism held no terrors for him, and he was slow to perceive the devilry of the Nazi creed.

Only when threatened with dismissal from his academic post did he join the flood of émigrés to England, though even then he was still sending his children on German holidays on the brink of war in 1939, and in touch with leading pro-Nazi art historians. His apparent obtuseness, Harries suggests in subtly analytical pages on his supposed fascistic inclinations, was due less to wilful blindness than to a lifelong political innocence and reluctance to cut ties with his homeland.

England proved a shock and, in social terms, a puzzle. Like Soviet Jewish pianists or violinists in Israel in later years, art historical refugees from Hitler were two a penny, and Pevsner endured years of penury and humble work, including as an adviser on household design ("the more art is applied to an article the worse its appearance becomes"), before his ascent to panjandrum status ("Is it in Pevsner?"), and eventual knighthood.

His success came not by social contacts  on the contrary, he was accused of having too few aristocratic acquaintances and of omitting grand country houses from his work for leftwing reasons  but by the manic diligence he was to show in the 23 years it took to compile the 46 volumes of The Buildings of England. He was most at home in churches, which he would root about tirelessly, "capital by bloody capital", though not entirely for spiritual reasons: "Really, the uses some people put these places to," he was heard to say when a service in progress obliged him to wait.

Culture clashes with the locals are entertainingly documented. In England art history was often an amateur affair, carried on with nonchalance, effortless superiority and class pretension, a place where folk such as John Betjeman (a modestly born social alpinist aware that his own name was of German origin) smirked about "Herr Doktor Professor", and where the very term Kunstforschung  art research  was thought frightfully amusing. "It was partly banter," Pevsner noted, "but not all banter." He was getting to understand the English.

Impressed nonetheless by innovative forms of popularising the arts in museums and lectures, under pressure from the BBC and others, he did his best to lighten the tone of his talks and articles, without succumbing to the personalised approach he found tiresome. Gradually his style, accent and all, found an audience, and numerous outlets, the Reith lectures included.

The feuds that assailed him, chiefly about his early book Pioneers of the Modern Movement, were one-sided affairs, in which he rarely hit back. Gropius was always his hero, which brought suspicions of continental theorising, inhuman functionalism and dangerous doctrines about the moral responsibilities of artists. He had definite, though unpredictable tastes, hating both brutalism and the flamboyant art deco of the Hoover building, and preferring more humdrum, workaday modern styles.

At the same time he involved himself in conservation battles, as postwar reconstruction, then 60s insouciance, conspired to obliterate outstanding Victorian buildings, and pulling down Covent Garden was seriously considered.

In the Nazi years it was better to be dépaysé abroad than in your own country, yet despite his English successes all his professional life you sense in Pevsner a certain homesickness. For us at least the conflict of national intellectual styles he represented was hugely beneficial. The irony of a "Prussian pedant" lecturing the English on Englishness, for which he was mocked, resolves itself in the fact that, together with Gombrich in art history and Weidenfeld in publishing, Pevsner was one of a golden generation of German/Austrian Jewish refugees who did much to give their adopted country the bottom it prided itself on already possessing.

Harries guides us through treacherous territory, of race, class, politics and artistic and intellectual intrigue, in a sure-footed manner. There is empathy with her subject, who had a kindly side (a "benign spider" someone called him), but her judgments are balanced by a cool and compendious intelligence, together with rare explanatory powers.

Intellectual movements, art politics, wartime history, a great man's unsteady emotional life  there is too much in this 800-page book even to evoke here. It is long because it is rich with things to tell and to say. A perfect blend of events, ideas and personal narrative, it is a masterpiece of the biographical genre 20 years in the making. As with much of Pevsner himself, no one, you feel, could have done it better.