"How had the pair of elderly Jewish lesbians survived the Nazis?” Janet Malcolm asks at the beginning of this extraordinary work of literary biography and investigative journalism. The pair, of course, is Gertrude Stein, the modernist master “whose charm was as conspicuous as her fatness” and “thin, plain, tense, sour” Alice B. Toklas, the “worker bee” who ministered to Stein’s needs throughout their forty-year expatriate “marriage.” As Malcolm pursues the truth of the couple’s charmed life in a village in Vichy France, her subject becomes the larger question of biographical truth. “The instability of human knowledge is one of our few certainties,” she writes.