Nonfiction -- briefly; A Day Late, by Carolyn Doty. New York: Viking. $10.95.

This book's merit lies not in the plot, which some readers will find too sordid for their liking, nor in the aura of catharsis and hope at the end, but rather in first-novelist Carolyn Doty's meticulous rendering of the moment-by-moment feelings of ordinary people. She writes with both delicacy and grit.

"A Day Late" opens with a certain studied craftmanship and an undue haste to give the crucial facts about its chief characters, a bereaved 41-year-old salesman on a business trip and a confused, pregnant teen-ager.

Sam Batinovich, obsessed with grief, guilt, and anger at the loss of his only daughter at 15, picks up Katy Daniels, a 17- year-old hitchhiker on a Nevada highway. Sam is aware of the irony in his befriending this troubled stranger, who reminds him of his daughter, and who makes him feel paternal, irritated, and sexually aroused in turn. He picks up a young man, then drops both. But not until he has found Katy again and has been her lover, as well as protector, can he begin to find a path out of his obsessive grief.

Despite this soap-operatic turn of plot, Doty woos the reader by her directness of perception and warmth of feeling, for her story. What has begun as something slender grows, under the dry heat and sudden storms of the desert, into something rich and strange. She is a newcomer whose literary talent bears watching.