Dubin's Lives, by Bernard Malamud. New York: Avon. $2.50.

William Dubin is facing a midlife crisis in rural New York, looking for but not finding comforting guidance from the subjects of biographies he's writing. He hopes diet and country walks will lift the burden of ennui, but his marriage is in drift and "The Passion of D. H. Lawrence" is still only on note cards. This novel, from an author who has won two National Book Awards and a Pulitzer, is in the end an affirmative view of coming to terms with problems.