Here is Philip Roth's beautiful, unsettling characterisation of his fellow American novelist: "like a moat so oceanic that you could not even see the great turreted and buttressed thing it had been dug to protect. You couldn't even find the drawbridge."

All great creative careers might, arguably, be described in this defended way, and perhaps all the biographer can do, if he can't catch the psychological drawbridge down for a moment, is to sail a few tentative raiding parties across the water in hope of a way in.

Zachary Leader has gone one step further. Volume one of his Saul Bellow biography is a vast 700-page pontoon thrown across the whole width of Bellow's career but concentrating here and for the moment on the first 50 years and leading up to the "fame and fortune" moment of Herzog.

It's an intriguing and ambiguous place to pause. For some, it remains Bellow's crowning achievement, a dense, learned, highly personal account of a mind in contact with a whole culture. For others, it's an oddly sophomoric book, dibbled together out of bits and pieces of casual learning, and driven by bilious anger and sexual revenge, its main target a former wife with whom Bellow had also warred in public.

Using it as a (temporary) terminus also seems to skid past potentially more important books, notably The Adventures Of Augie March which was to the 20th century in American letters what The Adventures Of Huckleberry Finn were to the 19th. In it, Bellow created a vivid new literary vernacular, and a mode of characterisation that leaves Roth's Alex Portnoy, Norman Mailer's Sergius O'Shaugnessy, or any of Bernard Malamud's gallery of self-portraitists seeming one-dimensional by comparison. Only John Updike's Rabbit Angstrom seems comparably attached to his rapidly changing culture.

Like Updike, Bellow is a great noticer, and a ruthless user of his own experience. In comparing multiple versions of the same events - some from the fiction, some from interview, some from external fact - Leader pieces together a great deal more than was previously known about Bellow's early life.

The family came to the US (illegally) from Russia via Canada, and settled in Chicago. Christopher Hitchens (just one of a claque of British writers who were openly obsessed with Bellow) suggests that the whole swing of the work from the wartime Dangling Man to the late short stories was an attempt to escape not just the city's Jewish ghetto, but a sequence of "ghetto psychoses".

One way out was through material success, which in the hectic mash-up of Chicago life, almost inevitably meant brushing sleeves with crime. This was the route taken by Bellow's older brother Maury, who is almost more vivid in Leader's account than in Bellow's own multiple versions.

The young man born Solomon Bellows may have had some contact with bootlegging. The other, more agreeable routes out were learning and sex, which seem to have been strangely conjoined in the future novelist's personality.

To know more than the next man and to be alluring to women of every kind offers something between escape and transcendence. Bellow's borderline erotomania can again usefully be compared to Roth's, Mailer's, Updike's. In all four it stands in some way for the desire to penetrate not just willing partners, but the great mysteries of the present.

Unusually, Bellow was supplied with a theoretical apparatus for this. More than any other commentator, Leader takes very seriously indeed Bellow's investment in the ideas and psychotherapy proposed by Wilhelm Reich, a figure whose reputation oscillates between modishness and airbrushed invisibility.

My own attempt to trace Reich's influence on American writers such as Mailer, Karl Shapiro, Paul Goodman, Anais Nin and, above all, Bellow, who also flirted with anthroposophy, sank without trace in 1983.

Perhaps the key texts in this are Bellow's Seize The Day in which Tommy Wilhelm's surrender to "oceanic" feelings beside the coffin of a man he didn't know are striking reminiscent of a Reichian breakthrough, and Henderson The Rain King in which an average-joe American, supposedly modelled on the novelist but made hulking and awkward to camouflage the resemblance, undergoes a highly explicit Reichian therapy while on safari in Africa. Untypically, Bellow had yet to visit the continent when he wrote about it and peopled it with wise natives in sometimes embarrassing blackface.

He himself had stood beside the coffin of Leon Trotsky, after the renegade Communist's assassination in Mexico. For a generation of American leftists, and particularly the group round Partisan Review, Trotsky was the embodiment of modern heroism.

Bellow was later to adapt the Russian's guiding principle by describing realism as the "permanent revolution" of modern literature, a principle he stuck close to, with the only partial exception of Henderson, which is a fantasy-fable.

What Leader brilliantly does is to unpick the knots of that realism, exploring how marriages, friendships, family connections were used, transformed, sometimes confounded in the novels and stories, often going back to drafts or to abandoned projects like The Crab And The Butterfly, one of the most discussed unpublished works in modern American writing.

In later years, Bellow became well-known and sometimes notorious for his fictional representations of real people, like the conservative philosopher Allan Bloom in Ravelstein or the poet Delmore Schwartz, another friend, in Humboldt's Gift, but for the moment it is the use of his own tangled marital history in Herzog that occupies the foreground.

To be accurate, Roth's moat image referred to Bellow's charm rather than to the man in total. We see much of that charm in Leader's account, but also the moments when it slips and flips into something less dignified or appealing.

Mailer and Roth have both undergone their public chastisements for conjugal violence, but Bellow stands no less accused here, his attitude to women slithering between reverence and angry contempt.

His second wife (of five) Sondra/Sandra/Sasha Tsachacbasov emerges as a highly complex and fascinating figure, far more various and appealing in Leader's version than in any of Bellow's, but also capable of maintaining her own narrative.

The destruction of a marriage and the weird euphoria that follows the emergence of a new relationship is part of the substance of Herzog whose "If I am out of my mind, it's all right with me, thought Moses Herzog" is the most arresting opening line since "Call me Ishmael".

Seven hundred pages in and still 40 years of life and writing to cover, Leader has already outstripped previous biographers James Atlas or Mark Harris, whose Drumlin Woodchuck is still a useful sketch.

But one wonders if the level of insight can be sustained, given that there is so much about the later career here already. It will be a quieter book, full of honours, but one senses that with fame and fortune the drama began to recede.

The Life Of Saul Bellow: To Fame And Fortune 1915-1964 by Zachary Leader Jonathan Cape, £35