A LONG DAY'S DYING (267 pp.)Frederick BuechnerKnopf ($3).
Frederick Buechner is a 23-year-old Princeton graduate ('47) about whom there will soon be lots of excited chatter in Manhattan avant-garde circles. His first novel, published this week, is a remarkably gifted and almost convincing display of virtuosity. At the opposite pole from such toughie novelists as Norman Mailer and Nelson Algren, Buechner employs the mannered, slightly effete and shimmeringly elegiac style of Henry James, which English Critic Cyril Connolly has aptly labeled Mandarin. Probably, Buechner is a forerunner of a spate of bright young Mandarins due to be turned out as highbrow critics...