Marx and Engels are arguably history's most famous couple. Such was the closeness of their collaboration that it is not always easy to recall which works bore both names, which just that of Marx, and which just Engels. Even when there was a single author, as in the most influential work of all, Das Kapital, Engels made a significant contribution to Marx's masterpiece. Marx, however, was always the lead player, the "first fiddle" in Engels's words, and, as a consequence, has been the subject of far more biographies. This is unfortunate: Marx without Engels would have been a significantly lesser figure; furthermore, Engels was an enormously capable, colourful, generous, kindly and engaging man, whose appetite for life in multifarious forms leaves the reader more than a little exhausted by the end of Tristram Hunt's book.

There are four central themes: Engels himself, the nature of his relationship with Marx, their personal involvement in the tumultuous events of the 19th century, and the evolution of their ideas. Like Marx, Engels came from a well-to-do Rhineland family. By the age of 19, Engels was rejecting religion and writing about the iniquities of industrial capitalism; soon after, he embraced Hegel, like most radical young German intellectuals of the time, including Marx. The two met for the first time in Berlin, where Engels spent a year doing military service. Soon after, he went to serve his apprenticeship in the Manchester branch of the family textiles firm, Ermen & Engels. If the heady debates of Berlin provided Engels with intellectual stimulation, Manchester, as the crucible of the world's first industrial revolution, was the place where he learned about the realities of industrial capitalism. Although a man of considerable means, he chose to spend most of his time with factory workers, in which he was much assisted by his relationship with Mary Burns, a semi-literate Irishwoman, who served as his guide to the other Manchester. It was an intimate relationship that was to last many years, but one Engels kept secret from his family and firm for fear of retribution.

His stay in Manchester bore fruit in the remarkable The Condition of the Working Class in England in 1844, a detailed and exacting study. Written at the age of 24, it remains a singular contribution to our understanding of the industrial revolution. On his return to Germany, Engels threw himself into the gathering struggles of the period leading up to the 1848 revolutions. He met Marx again, this time in Paris, and they rapidly forged an intimate relationship that was to last until Marx's death in 1883. From early on, Engels recognised Marx as his intellectual superior. He later wrote: "Marx was a genius: we others were at best talented ... Marx stood higher, saw further, and took a wider and quicker view than all the rest of us." Their involvement in these years was largely through the newspaper Rheinische Zeitung, which Marx edited. One is reminded that, in this apocalyptic period, revolutionaries lived a peripatetic existence, banished from one country to the next, constantly pursued by police spies. Eventually, as the revolutionary tide subsided, the two collaborators set sail for the safe haven of England, where they remained for the rest of their days.

Their personal paths were to mirror the changed political circumstances. After the convulsions and instability of the 1840s, capitalism now entered a period of growth and prosperity; the prospects of revolution disappeared beyond the horizon. The call to arms - most famously represented by their jointly authored Communist Manifesto of 1848 - gave way to a period of reflection and intellectual endeavour culminating, at the end of the 1860s, in the publication of volume I of Das Kapital

To this end, in an act of extraordinary self-denial, Engels returned to the family firm in Manchester to earn sufficient money to keep not only himself but Karl Marx and the entire Marx family. For Engels, it meant his effective withdrawal from the kind of political and intellectual activity that had been his chosen vocation. He continued to assist Marx in a multitude of ways, including furnishing him with the hard facts of capitalist production and frequently ghostwriting newspaper articles for the old Moor, as Marx was affectionately known to his family and intimates. But Engels was no recluse; the opposite in fact, a bon vivant of the first order. He was an enthusiastic womaniser, a connoisseur of wine, a remarkable linguist, a member of the highly exclusive Cheshire Hunt and the Brazenose Club, and a noted authority on military strategy. And he continued to live two lives, with two abodes: one with Mary Burns and her sister Lizzie, and one for bourgeois appearances. One of the most revealing things about Engels is the way, after the defeat of the 1848 revolutions, he chose to take a long walking holiday in the Loire Valley, mainly featuring fine wine and equally fine women.

As capitalism once more reveals how deeply unstable it is, and interest in Marxism revives, Hunt's biography is timely and very readable. He is perhaps at his best on the remarkable relationship between the two men; he also paints an endearing picture of Engels as a man it would be difficult not to like. Hunt clearly identifies warmly with his subject.

 Martin Jacques's When China Rules the World is published in June by Allen Lane

My boast that I am among the small number of people who have started to read Friedrich Engels' Anti-Dühring has to be qualified by the admission that I am also among the even smaller number of people who have not finished reading it. So I was distressed to discover, from Tristram Hunt's new biography of Engels, that what I found to be an unintelligible book is a "pacey, engaging and comprehensible explanation of the science of Marxism". Happily, Hunt's biography of Engels is clear and concise; indeed, he possesses a remarkable talent for explaining what is usually incomprehensible. That certainly includes dialectical materialism - "the critical tool for reading society's endless shifting contradictions and readiness for revolution which was Marx's definitive contribution to western thought".

In The Frock-Coated Communist, Hunt is helped to make the obscure plain by the assiduous use of quotations from Socialism: Utopian and Scientific, a rewrite of Anti-Dühring that is very nearly what we now call "a popular version". In it, Engels wrote that when the means of production become state property, "the proletariat abolishes itself as a proletariat, abolishes all class distinction and class antagonism, abolishes also the State as a State". I am still not sure how the thought-process that concludes with this fantasy can be called scientific rather than utopian, but, thanks to Hunt, my greater understanding of the general theory leaves me with one firm conviction: I am pro-Dühring.

History has made Engels appear the back end of the pantomime horse that produced The Communist Manifesto, with Marx at the front determining direction and speed. We learn from Hunt that although the seductive, heroic prose was pure Marx, "much of the hard intellectual grind... had been carried out by Engels". Without him, the call that "working men of all countries unite" would have been just more windy polemic. Industry and thought are what were to be expected from the author of The Condition of the Working Class in England in 1844, a survey that does far more than just report how the poor lived. When Engels, describing the Manchester slums, concludes that "only a physically degenerate race, robbed of all humanity, degraded, reduced morally and physically to bestiality, could feel comfortable and at home" in them, he clearly lacks the sympathy that, in later generations, motivated Charles Booth and Benjamin Seebohm Rowntree.

But unlike the earlier social scientists, he offered a comprehensive, if unattractive, method of righting the wrongs and, more important, an intellectually compelling analysis of how they came about. The Frock-Coated Communist brings Engels out from under Marx's shadow. That is the book's importance. Its attraction, as whoever chose the title realised, lies in the description of his origins and lifestyle.

Hunt tells the story with affectionate objectivity. Engels, the son of a pious and benevolent textile manufacture, was born in Barmen, Germany, in 1820. At school, the distinguished gymnasium at Elberfeld, he was attracted by "romantic patriotism". The enthusiasm for "Young Germany" did not last long. It took some time for him to decide what he really did believe. By the time he met Marx in 1842, he was a committed socialist of sorts, but not the sort of which Marx approved. His "distinctly chilly" reception was an unpropitious beginning to a partnership that changed the world. It survived Engels being sent to England, theoretically on behalf of the family firm but, in fact, to keep him away from radical company, and it endured despite the insatiate demands that Marx made on the man who became his benefactor. While Marx was working on Das Kapital in the British Museum, Engels's "toiling in the cotton trade [funded his] intellectual exertions".

And Marx kept asking Engels for more, even though he was better off than most members of the Victorian middle class. Engels was not a good manager. He found the labour theory more interesting than the price of groceries. This double irony - the theory of communism worked out at the expense of the working poor and the remedy for the world's economic ills prescribed by a financial incompetent - make a neat introduction to the moral question that Hunt's book poses. Should we care about a philosopher's lifestyle or are his ideas all that matters?

The word that best describes Engels's early manhood is "louche". But Hunt assures us that "the great Lothario, slave to Paris's finest grisettes and rough seducer... profoundly matured" by his early 60s. In the interim, he drank heavily. He also rode to hounds with the Cheshire Hunt. My hunting neighbours continually tell me that blood sports are a classless occupation. Yet I still find something ridiculous in the hero of Soviet intellectuals following a field led by the future Duke of Westminster - the unreadable chasing the uneatable.

He had moments of gentle concern, including the virtual adoption of Marx's illegitimate son when he was disowned by his father. But the virtue that shaped his life was the self-sacrificial affection he felt for Marx. The paradoxes of his life as cotton magnate and revolutionary socialist, as well as the complication of his theories, make his story difficult to tell. Tristram Hunt discharges the task with remarkable clarity.

 Roy Hattersley's Borrowed Time: The Story of Britain Between the Wars is published in paperback by Abacus.

Friedrich Engels: life of a Marx man

Born: 28 November 1820 in Barmen, Prussia, son of a textiles manufacturer. Died 5 August 1895 of throat cancer.

Education: Dropped out of high school for financial reasons.

Career: Joined the Prussian Household Artillery (1841); in 1842, started work in Manchester for family firm Ermen and Engels. Met Marx in Paris (1844) and they began writing together, first The Holy Family (1844). In 1845, moved with Marx to Brussels and wrote The Communist Manifesto (1848). Returned to Cologne that year, but fled on losing Prussian citizenship in 1849. Lived in Manchester and then in London (from 1870) to be nearer Marx. In 1880, Engels published Socialism: Utopian and Scientific

He said: "An ounce of action is worth a ton of theory."

They said: "The name and life of Engels should be known to every worker ... a great fighter and teacher of the proletariat!" - Lenin, 1896