WASHINGTON, July 28 — They were high school friends from Park Ridge, Ill., both high achievers headed East to college. John Peavoy was a bookish film buff bound for Princeton, Hillary Rodham a driven, civic-minded Republican going off to Wellesley. They were not especially close, but they found each other smart and “interesting” and said they would try to keep in touch. Which they did, prodigiously, exchanging dozens of letters between the late summer of 1965 and the spring of 1969. Ms. Rodham’s 30 dispatches are by turns angst-ridden and prosaic, glib and brooding, anguished and ebullient — a rare...