"The Benefactor," Susan Sontag's first book and first novel, originally published in 1963, introduced a unique writer to the world. In the form of a memoir by a latter-day Candide named Hippolyte, "The Benefactor" leads us on a kind of psychic Grand Tour, in which Hippolyte's violently imaginative dream life becomes indistinguishable from his surprising experiences in the 'real world.' Sontag's novel supplies a fascinating, knowing, acerbic portrait of a certain bohemian demimonde that flourished in France until quite recently. More important, "The Benefactor "is a novel about ideas-especially religious ideas-unlike any other: funny, acrobatic, disturbing, profound.