THERE ARE diaries and diaries. Andr,e Gide's is a collection of set pieces, and segments of these private reflections (containing precious little of his private life) were delivered regularly to the presses in the last dozen years of his life. Jean-Paul Sartre's diary was for Sartre first of all, thus closer to his deeper concerns -- his career, what people thought of him, the women in his life. He confesses that he used to hate diaries. But now, "when one is in the process of changing one's life like a snake sloughing its skin, one can look at that dead skin . . . and take one's bearings. After the war I shall no longer keep this diary, or if I do I shall no longer speak about myself in it."

A word about that title, War Diaries. The French original translates as Phoney War Diaries, referring to the period between the declaration of belligerency in September 1939 and the launching of Germany's blitz against France's northeast border in May of the following year. It was a time of considerable tranquility. Five miles behind the front, Sartre has the leisure to produce considerable quantities of prose. He is working on a novel, planning a major work of philosophy, and also turning out reams of this phoney-war diary, for the 358 pages which have been found reproduce only five out of 14 (or more) numbered notebooks, the others having been lost on a train. Sartre is also reading a lot: In the first two months of war he has read or reread 19 books. He gives us the list, and we find that it includes Kafka's The Castle and The Trial, Gide's hefty Journal, and half the plays of Shakespeare. He reads Goethe in the original, speculates about Heidegger's influence on his own thought, even employs German words occasionally (something few French soldiers were doing in their war diaries, we'd guess).

The Sartre we meet in these pages is already a ripe 34. He has published a novel, Nausea, a small book of stories, The Wall, a philosophical essay whose title could be translated as "Outline of a Theory of Emotions"; mainly he's a teacher of philosophy. He is not yet a famous writer, but before the war is over he will create the works that made his reputation, Being and Nothingness, his major philosophical statement, and his first produced plays, The Flies and No Exit (which German soldiers in Paris, and German critics, were to have the opportunity to applaud before we did). And while writing his diary he was also writing another novel, The Age of Reason, to be published shortly after the liberation of Paris.

SARTRE had been called up at the beginning of September 1939, sent east. He is in Alsace when we find him in mid-November in the opening pages of this book. When he isn't reading or writing he is a soldier in the meteorological service. After we leave him -- when the notebook breaks off in March -- he'll go to Paris, where his philosophical essay L'Imaginaire is being published, to receive a literary prize for The Wall. Shortly after that the Germans will sweep down from the Ardennes and Paris will fall. Although Sartre never manages to get close to combat he'll become a prisoner in June 1940, working his way out of the POW camp the following March thanks to some felicitous forgeries.

These War Diaries, which appeared in French in 1983, three years after Sartre's death, breach his intimacy for the first time. (His autobiographical The Words was revelatory -- but written for publication.) He discusses his relationship with friends, with lover and friend Simone de Beauvoir, with his army comrades. There are some painful passages in the tradition of confessional literature -- on his ugliness, and how he sought to deal with it by associating with beauty (beautiful girls as well as beauty in art). Funny moments too, as when he tries to cope with the dilemma of becoming a great man who must stay aloof, even from women, while realizing that "women certainly weren't running after me, indeed it was I who was running after them." He'd tell a young lady whose conquest he had just made not to infringe on his freedom. "But within a short space of time . . . I'd make her a gift of that precious freedom." His weight problem: "Every four or five months, I look at my stomach in a mirror and get unhappy." And so we are given what might be called the existential diet: "If I crack down on myself a bit roughly I have the impression of being my own master, hence free."

What we do not find in these notebooks, not even in embryo, is the political Sartre of the postwar decade. For, as astonishing as this may seem to readers who recall Sartre's public presence in the 1950s and '60s, neither the outbreak of war nor the troubled years which had preceded it -- years which saw the rise of Hitler, the Spanish Civil War, the anti-Fascist commitment of Sartre's friends in Paris -- had moved Sartre much. We know from Simone de Beauvoir's memoirs just how passive the Sartre-Beauvoir couple had been in the Popular Front years. Now in this diary he offers an imaginary reply to a critic who suggests that he might abandon literature for philosophy or "social preaching": there's no danger of that! "I feel no solidarity with anything, not even with myself." But if he has no social passion, if he lives outside his class and time, he doesn't necessarily admire this side of his character; he'd like to change. We now know that Sartre did get out of his armchair during the German occupation; he tried to get an intellectual resistance movement going, but in the face of indifference he gave it up. He ended the war contributing articles to the underground press, while making a reputation and possibly some money in German Paris. True political engagement would come later, often courageous, sometimes terribly wrongheaded, as when he moved toward the Stalinists at the very moment they were becoming ashamed of Stalinism.