THE SPANISH GAMBIT. By Stephen Hunter Crown. 389 pp. $15.95

LITERARY, Anglophiliac, constantly surprising, Baltimore Sun film critic Stephen Hunter's third novel is the thriller of the season. The setting is Barcelona in 1936-37. The Spanish Civil War is raging, and idealistic British youth are dashing south to support the Republican cause.

One such adventurer is the golden-haired aristocrat-poet, Julian Raines, overtly a foreign correspondent, covertly a spy for Moscow. Another arrival is Robert Florry, shy and gawky, Julian's most puppyish admirer while they were at Eton -- until graduation day, when Julian snubbed him in front of his parents. Now Florry is himself a spy, deployed by British intelligence to thwart Julian, whom he still both reveres and resents.

Vacillating between he two men is Sylvia Lilliford, an heiress who wants to experience "life." Masterminding the Moscow connection is Levitsky, "the Devil himself," an old revolutionary whose usefulness in the purge-ridden Moscow of the '30s is drawing to an end. Novelist Hunter shifts from one batch of characters to another with the assurance of a cutting-room pro.

One or two of the book's revelations may be stagey, and weapons and ways out have a handy habit of materializing just when needed. But for the most part, the plot keeps pace with the characters' developments. The style is lucid, the dialogue convincingly of its period, and the poetry Hunter has indited for Julian catches just the right note of waning lyrical energy.

In his acknowledgements Hunter cites obvious debts to the books and lives of Julian Bell and John Cornford, two English poets who died in Spain, and of George Orwell, who survived. The Spanish Gambit serves these and other literary forebears smashingly. BEFORE MY LIFE BEGAN. By Jay Neugeboren Simon and Schuster. 391 pp. $18.95

OCCASIONALLY a reviewer comes across a book that opens so promisingly and proceeds with such impassioned earnestness that he finds himself rooting for it to succeed. Jay Neugeboren's novel, Before My Life Began, is such a book. Its promise lies mainly in the depiction of a truly novel milieu -- that of Jewish gangsters.

His earnestness radiates from practically every line Neugeboren writes. Sometimes this intensity and his material mesh perfectly. Late in the novel he presents a mid-'60s confrontation between Mississippi rednecks and Northern civil rights workers. His charged prose and skillfully terse narration make the reader forget how many similar scenes he has witnessed in print and on film.

But for all its passion, Before My Life Began does not fulfill its potential. Neugeboren does little with the gangster trade and never quite accounts for the extraordinariness of his characters, who become thieves and hitmen -- my son the racketeer.

More disappointing, the central character, David Voloshin, is too enigmatic to bear the novel's considerable weight. This multi-talented man -- athlete of near-professional prowess, college graduate, artist, house-builder -- follows his adored Uncle Abe into a life of crime for reasons that are never made clear. After Voloshin kills a member of a rival Italian gang in self-defense, he discards his old life (and wife and kids) and adopts a new identity.

The Voloshin-to-Levin switch is another promise unfulfilled. Levin himself exposes the heart of the matter in a reverie: "He cannot imagine, no matter how good his new life seems, ever being able not to feel that he is the one who is hollow, that some part of him is missing." Much the same is true, alas, of Before My Life Began. RICHES AND HONOR. By Tom Hyman Viking. 448 pp. $17.95

TOM HYMAN'S third novel features an audacious and eerie imposture. In 1945, as American forces are conquering Germany, an SS guard at Dachau murders a Jewish patient and assumes his identity. The pseudo-Jew emigrates to the United States, marries a Jewish woman, accumulates an industrial fortune, and funnels money to the right places. As the contemporary action begins, William Grunwald (his stolen name) has been nominated as U.S. ambassador to Israel. Before he can be confirmed, he is kidnapped.

A parallel plot centers on Billy, the eldest Grunwald son, missing in Vietnam action for a dozen years. Everyone has his own idea of what's become of Billy: he's dead, he's imprisoned, he has been brainwashed, he has escaped but refused to come home.

Rob, his younger brother, fresh from a rebellious decade as a cocaine dealer and addict, mounts an expedition to rescue Billy. One of the novel's central questions is whether Rob can discard his easy-way-out mentality and become the lean- minded leader he would like to be. Meanwhile, scuttling behind the scenes, relying on his baleful genius and virtually omniscient computer, is the Pentagon's heliophobic Mr. Walker, unobtrusively manipulating most of the characters for his own -- and perhaps the U.S. government's -- ends.

Needless to say, Riches and Honor is full of action. Perhaps too full -- the book's last third is plot-heavy and vehicle- happy. But Hyman's strengths include character portrayal and motivation, and to his initial gimmick of Nazi-turned-Jew he brings not only understanding but compassion: Grunwald gets clear credit for having become a very good Jew manqu,e. Though it may be typical of the genre in its cynical wit and selfishly scheming personae, Riches and Honor is that rare bird, a thriller with a heart. THE TWO MRS. GRENVILLES. By Dominick Dunne Crown. 374pp. $14.95

AS JOE McGINNISS, Jonathan Coleman, Shana Alexander, and all the Claus von Bulow reporters can verify, nonfiction about intra-familial crime is all the rage. Why, then, has film producer Dominick Dunne chosen to put in novel form his recounting of what the tabloids called "the shooting of the century?" I'll hazard a guess in a few moments.

The original story is this. A beautiful gold-digging showgirl from Kansas named Ann lands a whopping socialite named William Woodward Jr. After a decade the marriage has frayed to the point of public shouts and slaps. One night in 1955 after a party honoring the Duchess of Windsor, Ann purportedly mistakes William for a prowler and shoots him dead. The family closes ranks, pulls strings, and gets the wife out from under everything but a cloud of suspicion.

Enter the ubiquitous Truman Capote. From his innumerable haut monde sources, he has pieced together a motive for murder. Capote makes use of this material in his much-ballyhooed but mostly chimerical manuscript, Answered Prayers; the relevant excerpt is published in Esquire. What the newspapers like to call a "tragedy" ensues for Ann Woodward.

Dunne disguises all of this so thinly that one wonders why he bothered. Truman Capote is Basil Plant, author not of Breakfast at Tiffany's but Candles at Lunch. Ann and William Woodward are Ann and William Grenville. On the other side of the looking glass, however, are real people -- the erstwhile Duchess, Elsa Maxwell, Brenda Frazier -- whom, for some reason, Dunne manipulates without benefit of pseudonym.

The novel never fails to entertain, contains some canny insights that Dunne may have needed the freedom of fiction to invent (early on William's sister thinks he won't marry Ann because she pronounces the t in "often"), but ultimately -- in its jumbling of fact and fiction -- fails to satisfy. Can it be that Dunne was so unsure of his facts that in the end he had to seek refuge in the license of fiction? FIRST LOYALTY. By Richard Lourie Harcourt Brace Jovanovich. 405 pp. $17.95

HERE IS another thriller, this one varying a theme by Ponce de Leon. The fountain of outh has become a drug, Stolat, developed by Soviet scientists and capable of lengthening human life by 30 years. Convinced that the substance is too hot to be hoarded by the Russians, some of the scientists try to smuggle its formula out of the country. Their ploy is to encode it on a film strip spliced onto the last reel of the Russian entry to the New York Film Festival.

At the same time a notable Russian poet defects to America -- except he is actually a plant. When the time is ripe, he will denounce the West and redefect, giving morale on the home front a dramatic boost. Both he and the film strip enter the life of David Aronow, a poorly-paid but feverishly busy translator of Russian poetry.

The author, Richard Lourie, himself a translator, writes admirable English. Particularly good is his description of one character standing in a doorway, "enjoying the feel of the quick mountain air, which had sudden cold places in it like lake water . . . " The plotting is firm, the Russian local color full-bodied. The characters, however, lack vibrancy. The pair of policemen assigned to a murder that figures in the case are stock coppers, the various women are hard to tell apart, and David Aronow is one more tired antihero.

Still, First Loyalty is worth reading merely for its foreigners' perspectives on Americans. They seem, to the defecting poet, "odd, futuristic, light, as if some component were missing from their makeup, as if the weight of the past had been lifted from their shoulders. So light."