"First it was all moral patterning. And felt life. Then it was all drugs and fucks. Now it's all tits and arses." This pithily reductive progress report on Martin Amis's new novel is spoken by a character in it, summing up not only her student boyfriend's increasingly boisterous approach to Eng lit, but also, The Pregnant Widow suggests, the unintended consequences of a cultural revolution. She's speaking in 1970  the year, as the narrator notes elsewhere, of Germaine Greer's The Female Eunuch and Kate Millett's Sexual Politics. Later in the decade, though not noted by the narrator, there will be The Rachel Papers, Dead Babies and Success: dispatches, as their author might now see it, from a battle of the sexes that was fought on unpredictably shifting terrain. Historical consciousness isn't something you'd automatically ­associate with Amis, but here he is with a long novel set in the times he started out in, a novel that's partly an advance post-mortem on his generation's historically constituted sexual selves.

The first time round, in the 70s, these were matters of some interest to Amis's literary generation, to which Keith Nearing, the new novel's central figure, also belongs. Ian McEwan, for example, read Greer's manifesto in 1971 and found it "a revelation"; his first two novels are, among other things, dreams about a collapse of male power. Amis, meanwhile, seemed to fit the sexual revolution into a wider sense of a world turning upside down, a sense that coarse, yobbish ways  ways that he was, as a good satirist, half in love with  were displacing high-minded talk about "felt life" and other literary-moral nostrums. Satirical inversions were his stock in trade, as in "It's Disgusting at Your Age", a slice of screenplay he published in 1976 and then cannibalised in Success. In it, 70s girls act like boys  swill pints, shag around, neglect their flats and appearances  while 70s boys swap grooming tips. "Posh girls," one such specimen disconsolately squeaks, "they're after one thing and one thing only."

Vivid yet analytically shallow ­reversals of this sort find a place in The Pregnant Widow. Early on, there's a riff on miniskirts, see-through blouses, knee-high patent leather boots "and all the other things you needed before you could act like a boy". There's also a good deal of chat about handjobs, boxes and so on: the dialogue of bookish, defensively cocky young men cracking wise among themselves, which was another of Amis's 70s specialities. This time, though, there's an enormous effort to put this behaviour in context, and the context isn't limited to feminism. Nuclear anxieties, the postwar "economic miracle", the fact of their not being called on  in contrast to parents and grandparents  to risk early death or widowhood: the narrator carefully threads all these into the book's depiction of "the generation less and less affectionately known as the Baby Boomers". There's also an ­effort to see things more squarely from the women's point of view  not an ­altogether successful one, maybe, but an effort nonetheless.

What all this boils down to, for the first half or so of the book, is a weird, slow-moving sexual comedy set in a posh girl's holiday castle in Italy. Keith  a short, chain-smoking would-be poet and orphaned adoptee brought up by academics  has been invited there for the summer with Lily, his down-to-earth, faintly mutinous and only reasonably attractive girlfriend. They're guests of Scheherazade, Lily's second-best friend, an upper-class English do-gooder. Having previously resembled "the girl who distinguished herself on the harpsichord, or clocked up five thousand miles for Meals on Wheels", she has suddenly blossomed into an ­incendiary beauty with, we understand, beyond-incredible breasts. Also on the scene are Whittaker, an older, gay American; his boyfriend Amen ("pronounced Ahmun"), who's Libyan and therefore  uh-oh  a Muslim; Gloria, Scheherazade's brother's uptight girlfriend; and Adriano, a tiny Italian toff. Most of them are 20 or thereabouts, and Scheherazade's drippy boyfriend is away.

Cutting at intervals to an older, sadder, three-times-married Keith in 2003, and announcing at the start that he'll have a life-alteringly traumatic sexual experience in 1970, the novel sets about asking the question: will Keith get to see Scheherazade's breasts? Then, once he's seen them, and the other characters have finished quizzing him about his response to this development, the question changes: will he manage to get his hands on them? A nice boy, not a natural schemer, Keith has brought a small library with him, and his studious reading for his degree course is used as a springboard for elaborate discussions of the parallels, or lack of them, between classic English novels and 70s mating rituals. Occasionally other characters drop by  notably a woman named Rita, who turns out to be such a fiercely liberated shagger that she has no time for love or even affection. (Amen's veiled sister, Ruaa, is brought on to serve as a dialectical counterweight to this terrifying figure, bringing an end to one Islam-related thread.)

As all this slowly happens, there's a growing sense that the reader is being asked to do too many things at once: to chuckle at the consciously puerile gags and over-literary running jokes, to nod along with the bulletins on ageing and baby-boomer sexual attitudes, and to attend solemnly to the busy surface of Amis's later style. Unless you're Christopher Hitchens, it's not easy to sustain the correct mood for doing all three simultaneously, and it doesn't help that Amis has expanded his repertoire of eccentric mannerisms. His Concise Oxford Dictionary has seen a lot of action (there are four etymologies in the first 50 pages, with at least 12 more to come), as has, I'd guess, his Oxford Book of English Verse. Above all, the need for each sentence to bear a heavy stylistic stamp often leads to such lines as "he insomniated by Lily's side", or "The clock, once in a blue moon, ticked. Or tocked. Or clocked. Or clicked, or clucked, or clacked." This section - the main trunk of the novel - is stilted, fiddling and rarely funny.

Around 250 pages in, however, as Keith's inept scheming builds to a catastrophe, the writing seems to relax a bit, collapsing the absurdly high diction into low comedy in more effective ways. It also starts to generate images reminiscent of the earlier, funnier Amis: "an extended dynasty of monogrammed leather suitcases", for example. Then, after several narrative twists, the Italian holiday abruptly ends and the writer seems to embark on a different book. Narrating in fast-forward Keith's sentimental education at the hands of a Nicola Six-like femme fatale in the 70s, 80s, 90s and beyond, Amis jettisons the lumberingly intricate, glassily poised manner of the earlier sections, working up instead a kind of narrative fugue state not seen in his work for some time. Although not every­thing succeeds here, you get the feeling of a writer working at high pressure with combustibly personal material, scarcely bothering to disguise various real-life figures  his sister, Hitchens, the poet Ian Hamilton. It's as though, having previously played only grace notes, he's launched into a tune.

A hostile reading of The Pregnant Widow might be that it blames Keith's moral quasi-degradation and failure as a poet on too close or too early an ­association with naughty ladies. A more sympathetic one would be that the novel portrays the 70s as the ground zero of a narcissistic baby-boomer culture that coarsened both sexes, a culture in which Amis's writerly enterprise is implicated too. (The narrator resurrects TS Eliot's notion of a "dissociation of sensibility" that cut thoughts off from feelings  a separation that's often been diagnosed in Amis's novels, not least by the narrator of The Rachel Papers.) There are fewer sage-like speeches on "universal" themes than in most of his recent fiction, and the minor character who functions as the book's feminist superego also seems to indicate that the novelist has seen through at least some of the Eurabia-type stuff he espoused not long ago. Is it a "return to form"? Not exactly, but there's plainly a regathering of artistic energies. It's a "strange ride with the pregnant widow", as the narrator says, and for stretches of it, the reader is happy to tag along.

For at least the past decade Martin Amis has seemed intent on making the most distinctive comic voice in contemporary British fiction  his own  do the most unlikely things. He's put it in the mouths of historical tyrants and 9/11 plotters, he tried it out for size  for laughs  as an impotent monarch and  in earnest  as a survivor of Soviet purges. He's had a go at Americans called Russia and women called He and one of the problems with all these characters is that they have sounded too smart, too Mart. The first thing to say about Amis's 12th novel, The Pregnant Widow, then, is that it is a great relief to find him back as a Keith.

The moniker might be a nod to Keith Talent, the antihero of Amis's last wholly successful novel, London Fields, but Keith is a homecoming for Amis in more than this sense. Keith Nearing is the most proximate a fictional alter ego he's written since Charles Highway in The Rachel Papers. This Keith is nearing 21 (his birthday, when our tale begins, is days away), he's nearing normal male height, like the author, "in that much ­disputed territory between five foot six and five foot seven", and he's inching toward a statuesque 20-year-old blonde named Sheherazade, with whom he is sharing a fabled summer in an Italian castle, along with several friends (including his ­semi-platonic and semi-liberated girlfriend, Lily).

Amis starts with a typically arch ­disclaimer, the suggestion that his tale  like the murder story in London Fields  is another "gift from real life". ­"Everything that follows is true," he drawls, blowing smoke at the reader. "The castle is true. The girls are all true, and the boys are all true. Not even the names have been changed. Why bother? To protect the innocent? There were no innocent" He has said elsewhere that the novel is "blindingly autobiographical" and, though names obviously have been changed, you half believe him.

We're mostly in 1970, at the moment when Amis himself started to find his voice. Few writers have ever been more conscious of ageing  like all prodigies he seemed totally undone by the creeping knowledge that even his dazzle would die  and having looked back on his lost youth first as crisis (in The Information), then as hard-won wisdom (in the memoir Experience), Amis finally, at 60, gives it a go as what it no doubt mostly was: romantic farce. The Pregnant Widow reminds you of those medieval epics in which the hero, Troilus, or ­whoever, observes from a heavenly vantage, free from earthly care, his teenage self ­tortured and dying for love, and permits himself more than a wry smile.

The version of his youth that Amis gives us here is a fleshed-out reincarnation of the narcissist he described briefly in Experience, "short-arseing along the King's Road" in green velvet flares, sending letters to Kingsley that concluded "Kafka is a fucking fool" or "Middlemarch is fucking good". "Aren't they nice, the young?" Keith's older self observes, here: "They have stayed up for two years drinking instant coffee together, and now they are opinionated  they have opinions."

In the castle Keith is cramming Eng Lit compulsively. He's ­force-feeding Richardson and Fielding, fast-forwarding Austen and George Eliot, each novel seeming to him a dramatisation of the interminable sexual frustrations he is experiencing around the castle's pool. Keith is a trier, and a dreamer (he's also, of course, a list-maker, an aphorism-coiner, and an italiciser); like Amis, he has swallowed Skeat's Etymological Dictionary whole and punctuates even his chat-up lines with lessons in linguistics. He is viewed by the author with amused and sometimes poignant affection ("Nostalgia, from Gk nostos 'return home' + algos 'pain' 'the return-home-pain of twenty years old'.") The portentous note that has sometimes been Amis's fatal flaw is mostly played here for comedy.

Consciously inhabiting the past, particularly this skewed slice of his own past, seems to liberate his writing from unwitting self-parody. He (and the reader) are spared the awkwardness of the last "big" novel, Yellow Dog, which seemed to be formed of a ­desperation to continue to accommodate what John Self once called (when Amis was really on the money) "the real stuff, the only stuff the present, the panting present". Looking back he knows every contour of the territory, the sex, the politics, the pretensions, and most of all the language. By ­framing his recollections in the present  it's not Keith that is speaking, we eventually learn, it's his grown-up conscience, the Jiminy Cricket of 2009 looking back on the Pinocchio of 1970  he finds he can have it all ways.

The result is a flashy Decameron of the sexual revolution; 20-year-old Keith may want to believe that his present moment  the Pill, female emancipation in the bedroom  has been plotted just for him, but a part of him can't help fearing he is on the wrong side of the barricades ("the Me Decade was the Me Decade, right enough  a new intensity of self-absorption. But the Me decade was also and unquestionably the She Decade..."). Women  in particular the women Keith observes in torturous peripheral vision plunging in and out of the castle's pool, topless (and occasionally bottomless)  are undoubtedly more available in theory, but not, strictly, in his experience, in practice. Keith is doomed and hamstrung in his pursuit of Sheherazade not only by his legion of neuroses, and a vestige of old-fashioned loyalty to Lily, but also by rival suitors  an absent (and very tall) Pentecostalist, and an ever-present (and very short) Italian count. Love, in 1970, appears to have been replaced by "hysterical sex" and of course "hysterical sex means never having to say you're sorry".

Tragically and despite all of his historical advantages, it appears Keith's own strike rate as a result won't improve on Samuel Richardson's Lovelace in Clarissa ("one fuck in 2,000 pages," he notes glumly), and predictably this is the source of much bathetic torment, delivered with all Amis's mastery of register and tone. Unusually for Amis, Keith's deferred gratification also injects into the novel that other, often elusive, 18th-century quality, suspense ("Amis novel" and "page-turner" have not always been synonymous). There are other surprises, in comparison with recent Amis, too: fully realised female characters  Lily, in particular, Keith's almost cynical ­girlfriend, is shown torn between having it all and having nothing at all; and walk-ons who are not just one-liners (Adriano, the diminutive count, is a ­virtuoso ­performance).

For the most part Amis stays within the limits of this comedy of manners; when he is finally tempted to stray beyond it in the latter third of the book, with the introduction of the girl Keith eventually does get, and regret, his substitute Sheherazade, Gloria Beautyman, the plotting creaks just slightly. Beautyman spins Keith seductive yarns about her age, and her religion, truths that are unveiled in an ending that strains for universal significance. This intervention can be forgiven, though, in some vintage Amis peacockery: riffs on the earthiness of Italian plumbing and the obviousness of Italian men, on Montaigne and Northanger Abbey, and fresh updates on such familiar refrains as hangovers ("The air itself was about to throw up. And he could hear the yellow birds in their tree  pissing themselves laughing") or the evolutionary insistence of winged insects, those "armoured survivalists with gas-mask faces".

For a long while, it has been hard to imagine how a writer much concerned with reputation would begin to fashion for himself a convincing late period to match his stellar youth. This novel looks a lot like one answer to that. Amis has, of late, become a professor of creative writing at Manchester University and you could even begin to imagine that his position has prompted a satisfying return to first principles. Lesson number one: always write what you know.