By the time she died four years ago, Susan Sontag had been for decades a kind of intellectual plenipotentiary, novelist, culture critic and that most unlikely of all job categories, famous essayist. In person, Sontag could be warm, patient and funny. On the page, she was omniscient and intimidating, somebody who had read everything and assumed you had too. Where, you wondered, did she find the time?
Now we know: she got an early start. Reborn, Journals & Notebooks, 1947-1963 (Farrar, Straus & Giroux; 318 pages), the first of three projected volumes selected from the diaries Sontag kept nearly...