THE WHITE ALBUM By Joan Didion (Noonday: $7.95) . Joan Didion's 1979 collection of essays is the perfect book read in tandem with "An Island on the Land." Her descriptions of the moods and quirks of life in contemporary Los Angeles complement McWilliams' account of their antecedents. The subjects of her reflections range from a recording session by the Doors and the public reaction to the Tate-LaBianca murders to Hollywood deal-making, the Black Panther Party and the unoccupied, elephantine Governor's Mansion in Sacramento. As an essayist, Didion lacks the hyaline profundity of Susan Sontag or the classical erudition of Marguerite Yourcenar, but her stance as "a woman who somewhere along the line misplaced whatever slight faith she ever had in the social contract" makes her the perfect chronicler of life in the "have a good one" city, where everyone observes the letter of that contract--and violates its spirit.