We first meet John Milk, the clueless, conflicted narrator of T.C. Boyle's terrific 10th novel, The Inner Circle, in 1956 as he mourns the death of his mentor, the brilliant, charismatic, and profoundly creepy sex researcher Alfred Kinsey. (Though the novel is based on real events, all the major characters except Kinsey and his wife are fictional.) Locked in his study, Milk finishes off a midday zombie cocktail and reminisces into a tape recorder about Prok, as Kinsey was known, while his contemptuous wife, Iris, bangs on the door. ''Jesus, you make me sick,'' she says. ''He wasn't God, you know.''
Actually, that's exactly what Prok was to Milk. Unfolding in flashback, Boyle's book offers a blistering portrait of Kinsey -- bow-tie-wearing genius, priapic weirdo, and manipulator of the first order -- filtered through the adoring eyes of a born follower. Zealots never fare well at Boyle's hand, and Prok, who was fixated on the clinical details of sex while utterly blind to its emotional ramifications, looms as one of the most wonderfully repellent figures in recent literature.
In 1939, Milk, a virginal Indiana University undergrad, enrolls in the racy ''marriage course'' taught by the notorious zoology professor Kinsey. Though married with three children, Prok espouses a radical free-love philosophy and likes to shock his Bloomington neighbors by wearing only a flesh-colored jockstrap while gardening. Prok has recently begun collecting personal histories for a monumental study of sexuality, and he urges students to pony up their secrets ''for the sake of science.'' And so, Milk finds himself in Prok's office revealing his masturbation habits. He walks out with instructions to measure his penis -- flaccid, erect, and ''oh, yes, if you would just note the angle of curvature as well.''
In the prim prewar era, Prok's matter-of-fact approach to sex has some obvious allure; Milk signs on to help with the professor's research, becoming the first member of Prok's ''inner circle'' -- a small group of trusted young men who gather thousands of explicit histories, participate in ever more bizarre sex experiments, and periodically share Prok's bed.
The last part of the job becomes problematic for Milk after he marries Iris McAuliffe, a pretty, wholesome girl from his Mid-western hometown. (''What's the matter, you're not getting sex shy on me, are you?'' Prok asks when the newlywed Milk temporarily resists his advances.) Iris, understandably, casts a skeptical eye on her husband's boss. ''The whole world hangs in the balance, and you're out there somewhere in the hinterlands measuring orgasms,'' she says to Milk during World War II. ''I mean, doesn't that strike you as trivial?''
But little seems trivial to the good sex soldier Milk, even filming a thousand men as they masturbate on a tarp to determine how many ''dribbled'' and how many ''spurt.'' Though it takes a while to figure this out, ''Circle'''s hero is smart, commonsensical Iris, who understands that Prok's mechanistic vision of sex fails to account for love, jealousy, and human nature. Iris goes to excruciating pains to communicate Prok's fatal flaw to the laughably dense Milk, who still can't really comprehend that his idol may be a monster. Readers, however, will have no trouble coming to that conclusion.
Boyle has never been one to pull his punches. From the frigid health-food nuts of 1993's ''The Road to Wellville'' to the selfish hippies of last year's excellent ''Drop City,'' Boyle has created a gallery of great, memorably loathsome characters. Now comes Prok. No one -- not Sherwood Anderson, not Francisco de Goya, not David Lynch -- is better at capturing a grotesque.