IAN McEWAN'S work falls into two distinct periods. His early stories and novels were all cool post-1960s perversity, a high-end parade of deadpan macabre and kink and sideshow eccentricity: ghastly death, corpses and butchery, bestiality, incest and pedophilia, insanity, dwarves. But since he turned 50, around the turn of the century, he's published lovely historical fiction about the disastrous sexual misunderstandings of youth ("Atonement," "On Chesil Beach"), and contemporary fiction about an alternative-energy researcher ("Solar") and a deeply sane, happily married surgeon ("Saturday"). It's as if Johnny Rotten had changed into Bono. And in the same way I like both the Sex Pistols and U2, I've enjoyed the best of McEwan's fiction in both modes. "Sweet Tooth," his new novel, is definitely mature McEwan, intermittently funny and much more sweet than bitter, about as entertaining as a very intelligent novel can be and vice versa. Even though the story is set inside a cold war espionage operation, no violence occurs - indeed, only one (secondary) character dies, of natural causes, and only after he's exited the story. The narrator and heroine, Serena Frome, is the elder daughter of an Anglican bishop who, she says, "I don't think . . . had ever been in a shop." "Nothing strange or terrible happened to me during my first 18 years" in the 1950s and '60s, "and that is why I'll skip them." She "was both clever and beautiful," and reminds us again 10 pages later: "I really was pretty." Her mother persuades her to fulfill her "duty as a woman to go to Cambridge to study maths," where she promptly learns "what a mediocrity I was in mathematics." What Serena really enjoys is reading fiction. "Reading was my way of not thinking. I didn't bother much with themes or felicitous phrases and skipped fine descriptions of weather, landscapes and interiors. I wanted characters I could believe in." Her tastes are defiantly un-snobbish: she amuses university friends with her insistence that "'Valley of the Dolls' was as good as anything Jane Austen ever wrote," and she discovers Solzhenitsyn right after reading Ian Fleming's "Octopussy." Although she "took the orthodox view of our generation" concerning the Vietnam War, the fiction she reads turns her into a young anti-Communist in the soft-on-Communism academia of the early '70s. "I was also the first person in the world to understand Orwell's 'Nineteen Eighty-Four.'" She's not quite Emma Bovary, ruined by the fiction she inhales, but "those books delivered me to my career in intelligence." She has a brief affair with a middle-aged history tutor who in turn gets her recruited by MI5, the domestic counterespionage service. Compared with the lavish attention McEwan often devotes to physical description, "Sweet Tooth" is light on telling period detail. "It pleased us, the general excitement in the air in 1969," Serena says early on, and again, not many pages later: "A seedy, careless insurrection was in the air." But we're mainly obliged to take the countercultural atmosphere on faith, with the exception of some funny passages involving Serena's hippie sister, Lucy, who lives "rent-free with another woman, a circus-skills instructor." "Without asking too many impertinent questions, the state paid the rent and granted a weekly pension to artists, out-of-work actors, musicians, mystics, therapists and a network of citizens for whom smoking cannabis and talking about it was an engrossing profession." Lucy's boyfriend is one of these, doing "that inexcusable thing that men who liked cannabis tended to do, which was to go on about it . . . Our parents had the war to be boring about. We had this." Yet Serena's distaste for "this inglorious revolution" is more a matter of sensibility than ideology; she is a young fogy on instinct rather than principle. "I believed in nothing much - not carols, not even rock music" Against the conformist nonconformity of her fellow youth, she enjoys being a (secret) nonconformist. "It gave me some innocent pleasure to think how horrified the counterculture crowd around us would be, to know that we were the ultimate enemy from the 'straight' gray world of MI5." Organizing an undercover operation code-named Sweet Tooth, this fictional MI5 contrives to pay long-term stipends, through a front foundation, to 10 up-and-coming writers. They didn't need to be cold war fanatics, merely "skeptical about utopias in the East or looming catastrophe in the West." The hope, one of the bosses tells her, is that they'd "turn out well and become, you know, important. This is a slow-burn thing." Because Serena knows contemporary literature, she gets the assignment to recruit the young fiction writer Tom Haley. They promptly begin an affair and fall in love. She keeps him in the dark about his true patrons. Meanwhile, her adulterous "old MI5 hand" turns out to have been a Communist asset, putting his protégé Serena under suspicion. As in any spy story, it's unclear who's lying to whom until late in the game. For all the modish noir of his early work, McEwan has always been a good old-fashioned teller of tales, and the suspense and surprises in this book are well engineered. Most big-time novelists sooner or later write a novel or two about books and writers, and this is not McEwan's first iteration. Its true subject is not espionage but, as in "Atonement," the porous boundaries between the imaginary and the real -and, as in "Atonement," he's got a large metafictional trick up his sleeve. In other words, if I may indulge in my own metanonfictional swerve, "Sweet Tooth" is "a novel about the powerful influence literature can exert on life" - as a reviewer last summer wrote in these pages about my own latest book, also a circa-1970 story concerning an upper-middle-class fiction-besotted baby boomer girl who reads Ian Fleming and plays at espionage with duplicitous friends, also narrated by the rueful heroine four decades later. Serena tells Tom (and us) again and again that she has no use for the illusion-busting postmodern novelists he adores. "I wasn't impressed by those writers . . . who infiltrated their own pages as part of the cast, determined to remind the poor reader that all the characters and even they themselves were pure inventions I believed that writers were paid to pretend." And, later: "No single element of an imagined world or any of its characters should be allowed to dissolve on authorial whim. The invented had to be as solid and self-consistent as the actual." McEwan, however, has his cake and eats it, until the last chapter keeping us unaware of the metafictional con under way. Instead of flaunting it, in 20th-century spoilsport fashion, he uses his game to reinforce and deepen the pleasurable illusions of reality, thereby satisfying conservative readers like Serena as well as those like Tom with a taste for the Jiterary fun house. Even before the reveal, "Sweet Tooth" playfully hops and skips along the borders of make-believe and reality. Unlike her coworkers, who tell family and friends they work for MI5, Serena unnecessarily gives a cover story, turning herself into a kind of fictional character. A colleague warns her that in intelligence work "the line between what people imagine and what's actually the case can get very blurred. ... You imagine things - and you can make them come true." She's happy to indulge Tom's masochistic sexual conceit that she's cuckolding him with Saul Bellow and Norman Mailer, "a deliberate and shared fantasy . . . usefully diluting my own necessary untruths." But sex with a writer also unnerves her: "I couldn't banish the thought that he was quietly recording our lovemaking for future use, that he was making mental notes." McEWAN studs the novel with well-known Britons, both named (his former publisher, his former editor, his friend Martin Amis) and lightly fictionalized. The future MI5 director Stella Rimington is "Millie Trimingham"; the book's ambitious undergraduate editor Rona Kemp ("She went on to Vogue . . . and then to an incendiary rise and fall, starting new magazines in Manhattan") seems highly Tina Brownian; and Tom Haley is almost indistinguishable from McEwan himself. Serena summarizes a half dozen of Haley's short stories, several of which are recognizable as versions of McEwan's fiction from the 1970s. "Sweet Tooth" is sort of a younger sibling to "Atonement," less epic and grave, with lower stakes, more fun and an apparently happier ending. Tom is a self-consciously autobiographical figure, but one throwaway line of Serena's - "And feeling clever, I've always thought, is just a sigh away from being cheerful" - could be an animating truth for McEwan as a writer. "Sweet Tooth" is extremely clever in both the British and American senses (smart as well at amusingly tricky) and his most cheerful book by far. Kurt Andersen is the author, most recently, of the novel "True Believers."