The ruthlessness with which Simone de Beauvoir documents Sartre's deterioration is, at first, appalling. The puddle of piss he leaves on a chair is recorded. So is the dribble on his shirt. Nothing is shameful to de Beauvoir if it is true: the ugliest, the least dignified truth is beauty.

The staccato rat-a-tat of the years of Sartre's faltering final decade, 1970–1980, shatters our and the 19th-century's obsession with immutable Grecian urns, with adolescent 'perfection', with euphemism. This book is an extraordinary achievement, precisely the right encomium for a man whose passion was mind….

Neither [de Beauvoir nor Sartre] ever shies from truth, even when the terse, almost too scrupulous diary which makes up the first third of this 450-page book, and which mentions even the deathbed gangrene and her own drunkenness, gives way to the conversations.

Taped in 1974 after blindness made it impossible for him to write, they...