John Knowles, 75, author of the classic 1959 novel "A Separate Peace," which explored the adolescent relationship and rivalry between two friends as well as the atmospherics of their fictitious East Coast prep school, died Nov. 29 at a nursing home in Fort Lauderdale, Fla. The cause of death was not reported.

Mr. Knowles was only 34 when he published his first and most-acclaimed novel, drawing heavily on his experiences at Exeter Academy in New Hampshire. Generations of students have read the opening lines, "I went back to Devon School not long ago, and found it looking oddly newer than when I was a student there fifteen years before."

The story describes the idyllic life at the school, where a "separate peace" reigned as the country was swept into World War II. The narrator, the bookish and conscientious Gene Forrester, becomes increasingly resentful of his athletic friend Phineas as "Finny" becomes more popular. Gene causes a crippling injury to Finny when he pushes him from a tree. Finny, after a subsequent injury, dies during surgery. Those events and his apparent complicity in Finny's death are a springboard to introspection for Gene.

Writing in The Washington Post in 1981, reviewer Paul Piazza described the novel as having "movingly chronicled the struggle between two adolescents who, too young to enlist, discover the enemy not in Europe or in the Pacific, but in themselves."

Because of its relatively short length, its rich store of identifiable conflicts and contrasts and its deceptively simple language, the book quickly became part of junior and high school curricula, avoiding the controversies swirling around harder-edged novels about adolescence.

The book, for which Mr. Knowles received the William Faulkner Foundation Award, was made into a 1972 movie, directed by Larry Peerce and starring Parker Stevenson and John Heyl.

Although Mr. Knowles wrote eight other novels, as well as a collection of short stories and a volume of travel pieces, none of his works attained the critical acclaim of "A Separate Peace."

He went back to Devon in 1981 with "Peace Breaks Out," which the reviewer Piazza said lacked "the power and tightly wrought structure of his earlier work." His last novel, "The Private Life of Axie Reed," was published in 1986.

Mr. Knowles was born in Fairmont, W.Va., where his father was a coal company executive. He left for Exeter when he was 15. He was a graduate of Yale University and, working as a journalist and freelance writer, came to the attention to playwright Thornton Wilder.

He taught at Princeton University and the University of North Carolina before moving in the 1980s from Long Island, N.Y., to the Fort Lauderdale area. He had taught creative writing at Florida Atlantic University in Boca Raton.

Survivors include two sisters and a brother.