More than comfortably rich, crippled by a youthful accident but able to swing down the streets with brace and crutch (and swing in bed quite nicely too), Marshall Lewis Henderson--an amateur scholar of the Albigensian heresy--cossets his boredom with real flair. His newest idea is to buy an old manse in Tatter-hummock County, Va., that once belonged to his mother's family--not an especially good idea, perhaps, but worth a shot. As Marshall sits up in New York waiting for the Virginia house to be his, the background deepens with memories of his embezzler father, his brother (a suicide), and his serially-espoused mother. Also--flashbacks to his prep-school days and first sex with a black girl (by whom Marshall fathered a black-power figure who would probably lose control of his clenched fist if he ever found out who his papa really is). And musings over his travails with current mistress Jackie, a trendy freelance journalist. In Marshall, Canby has both a stationary and, when necessary, moveable goad: the book's sharp social alertness (packed with proper nouns and press parodies--not surprising from Times-man Canby) is consistently amusing, and the touch is light and stinging. ""As I like to say,"" Marshall confesses, ""when I've been especially rotten and am in need of instant absolution, I know I'm not always a nice person."" The joys of being nasty--and, for the most part, getting away with it--in a pungent and deft and funny book.