THE SOUL OF VIKTOR TRONKO By David Quammen Doubleday. 350 pp. $17.95

There is no Dmitri, insists Viktor Tronko -- he of this book's title -- after defecting to the West from his post high in the KGB. That's not all he says, of course. He talks about Lee Harvey Oswald and, because the year is 1964 and the Warren Commission is toiling away, he is listened to. But his cancellation of Dmitri serves as the pivot point for David Quammen's new spy novel.

Dmitri, you see, is an alleged traitor inside the CIA, one of the top people, ferreted not quite all the way out by a previous KGB defector, Bogdan Fedorenko, who from inside Russia had been able to infer that such a mole existed but not who he was. For convenience around the agency, the mole was given the name Dmitri. Before he died Fedorenko had predicted Tronko -- not by name but by function. The Soviets will send you someone to discredit me, Fedorenko had warned, someone who will try to persuade you that there is no Dmitri, someone who, in short, will sow disinformation.

Now there is a right way to sow and a wrong way. Tronko does not enter chattering, "I'm Tronko, I want to defect, there's no Santa Claus and no Dmitri, either." Rather, by displaying intricate knowledge of the KGB at its highest levels, Tronko sets himself up as a party in the perfect spot to have known about Dmitri. But when asked, he looks blank and replies, "Who?"

The CIA devotes several person-years' worth of its most talented agents to gauging Tronko's veracity. One contingent questions him using techniques that stop just short of torture but does not succeed in breaking him. Given a polygraph test, he foils it not in the sociopathic manner of soaring above both honest answers and blatant lies with utter sang-froid but by seeming to lie constantly and unabashedly, nudging the needle up to ever-higher spikes of mendacity.

Exasperated, the agency assigns a second contingent to the analysis of Tronko's transcripts. For months they read, ponder and meet, never laying eyes on Tronko himself, read and ponder some more. Searching for inconsistencies among the reams of paper devoted to Tronko's answers, they try to determine his legitimacy once and for all.

This is a fetching premise for a thriller: the spectacle of one side's best minds trying to outthink the other's -- a premise used to brilliant effect in Robert Littell's novel, "The Defection of A.J. Lewinter." Unfortunately David Quammen does not present it nearly as straightforwardly as I have here. "The Soul of Viktor Tronko" is set in the early 1980s, when journalist Michael Kessler learns of the Dmitri enigma from a CIA friend who is murdered before he can finish divulging it. The novel proceeds via long conversations with retired agents, each of whom insists on depositing an arabesque of background and only then going on to answer Kessler's blunt questions. One fastidious raconteur insists that he can't tell Kessler even a condensed version of what he knows in fewer than three days. During that period Kessler questions other sources, and Quammen's technique frequently leaves multiple skeins of narrative dangling. It's the sort of novel whose front endpaper does double duty as a blackboard for readers keeping track of the plot.

Though it takes stamina, staying with "The Soul of Viktor Tronko" does pay off. Quammen, whose natural-history essays for Audubon and Outside have brought him deserved kudos, writes posh prose, as in this characterization of Kennedy-assassination fanatics: "Every coincidence is sinister, for these people. Every loose thread in even the wooliest snarl of random and contradictory data makes them smirk knowingly, not to be fooled. Meanwhile, they themselves float free of rational tethering, riding all the more strangely ionized breezes." And he depicts violent action, much of it set in the Washington area, with a freshness that old hands might emulate. Finally, he solves the riddle of Dmitri deftly and surprisingly.

One can understand why Quammen located the crux of his tale so far in the past: Still-working CIA agents are not wont to bare their souls, let alone the souls of defectors, before journalists. But I can't help wishing that, next time, this talented writer will put his impressive mind to the task of fashioning a more immediate thriller. The reviewer is a Washington writer and editor.