''I don''t believe in God, but I miss Him.'' Julian Barnes'' new book is, among many things, a family memoir, an exchange with his philosopher brother, a meditation on mortality and the fear of death, a celebration of art, an argument with and about God, and a homage to the French writer Jules Renard. Though he warns us that ''this is not my autobiography'', the result is a tour of the mind of one of our most brilliant writers.