THIS IS William Styron's fifth novel, and the narrator, nicknamed Stingo, is very like Styron himself 30 years ago when he was an intensely ambitious serious novelist. In the summer of 1947 Stingo is a book and music-crazed graduate of Duke, a recent noncombat Marine officer, and an aspirant to fiction. Thrown out of his Manhattan publishing job for impertinence, he settles in a Brooklyn rooming house and lays out his yellow pads and Venus pencils, determined to write an important "Southern" novel.

There he is taken up by two memorable characters: Nathan Landau, a manic, madly enteraining Jew posing as a distinguished research chemist and Sophie Zawistowska, a blonde Polish girl late of Auschwitz, whom Nathan has met in the Brooklyn College library, helped restore to health, and fallen violently in love with. Although a Virginian whom Nathan loves to bait for presumed cruelty to blacks, Stingo is drawn into their world, appalled and facinated by their furious lovemaking overhead and their equally audible and physical quarrels.

In the course of the summer, Nathan's furies grow lunatic, and Sitngo discovers that his wild friend is hopelessly, murderously insane. He discovers all about Sophie, too, as over the summer she gradually confesses. She has been sent to Auschwitz not for Jewishness but for smuggling a bit of meat in Cracow. Actually, she comes from a vigorous anti-Semitic Polish line, and her university professor father has published an earnest pamphlet (Poland's Jewish Problem: Does National Socialism Have the Answer? ) proposing the Final Solution well before the idea has caught on in Germany. At Auschwitz she becomes secretary and Polish translator to the commandant, Rudolf Hoess, living in his house with other trusties and collaborating to secure her own safety and that of that of her son.

The syndrome of survivor's guilt which impels her to confess to Stingo is familiar, but her guilt is special because she is not a Jew. And in the mad world of Auschwitz, she has found that even the most abject collaboration has not brought its assumed rewards. Sophie's choice has been offered her upon arrival at the camp with her two children. On the selection platform a drunken SS officer grants her the choice of one of her children. She chooses the boy, and the girl is pulled immediately into the gassing line. The son is taken to the Children's Camp, and she never sees him again either. Sophie's choice has been a sick joke, the SS version of Catch-22.

"I don't think I'm going to make it," she tells Stingo near the end of her agonizing summer of recall, and finally in the heat she and Nathan kill themselves, she out of a rational guilt, he out of sheer lunacy. It has been a summer in which Stingo has learned "at firsthand and for the first time in his grown-up life about death, and pain, and loss, and the appalling enigma of human existence." He proposes the question, "At Auschwitz, tell me, where was God?" and sometimes the novel dwells on that question. But it does not answer it. Rather it is replaced with a question less theological than moral: "Where was man?"

"This modern Gothic freak," Styron calls the commandant of Auschwitz. Styron's achievement in Sophie's Choice is to infuse new life into the tradition of Southern Gothic fiction running from Poe to Faulkner and William Goyen. He does this by locating in the systematic destruction of the European Jews an action adequate to the genre's ripest conventions of madness, perversity, cruelty and guilt. Life imitates art, and Auschwitz replaces Yoknapatawpha County. Styron dramatizes the social and political forces that in the '50s and '60s are going to displace "the Southern novel" in favor of "the Jewish novel." In Sophie's Choice Styron becomes an honorary Jew Without ceding a bit of the Southernness that shows him the parallel between black slavery and Polish anti-Semitism and instructs him in the guilt appropriate to both. But for all its Southernness, Sophie's Choice is in the main stream of the American novel. Like The Portrait of a Lady or The Great Gatsby , it is about the innocent American's discovery of immitigable evil and his complicity in it. (While the assembly-line gassings were taking place over there, over here Stingo was eating hanburgers and drinking Cokes and going to the beach and "dating.")

Sophie's Choice is wonderfully human, which means that for all the guilt and gloom, it offers splendid comedy too. Styron projects Stingo comically as a callow virgin in the '40s style, like Alexander Portnoy a whacker-off and the constant victim of foul-mouthed girl teases who always withold the ultimate favor. Styron's first chapter, detailing Stingo's fall from grace as a manuscript reader at McGraw-Hill, is as funny in its closely observed detail as anything in recent fiction. An index of Styron's skill is his proceeding from that kind of outrageous social comedy smoothly and naturally to the horrors of Sophie's and Nathan's predicaments and to the final scene's new but plausible key:

"The tears finally spilled forth . . . tears . . . I had tried manfully to resist and could resist no longer, having kept them so bottled up that now, almost alarmingly, they drained out in warm rivulets between my fingers. . . . I did not weep for the six million Jews or the two million Poles or the one million Serbs or the five million Russians - I was unprepared to weep for these others who in one way or another had become dear to me, and my sobs made an unashamed racket across the abandoned beach: then I had no more tears to shed, and I lowered myself to the sand on legs that suddenly seemed strangely frail and rickety for man of twenty-two." If that's not quite as good as the ending of The Great Gatsby , it's fit to be brought next to it.

Sophie's Choice is Styron's most impressive performance so far. It be longs on that small shelf reserved for American masterpieces.