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 unfiltered look into the head and heart of a future first lady and would-be president. Their private expressiveness stands in sharp contrast to the ever-disciplined political persona she presents to the public now.

“Since Xmas vacation, I’ve gone through three and a half metamorphoses and am beginning to feel as though there is a smorgasbord of personalities spread before me,” Ms. Rodham wrote to Mr. Peavoy in April 1967. “So far, I’ve used alienated academic, involved pseudo-hippie, educational and social reformer and one-half of withdrawn simplicity.”