It was a rescue operation for an unlikely victim: 16th-century astronomer Nicolaus Copernicus. After reading critic Arthur Koestler's almost casual dismissal of Copernicus' groundbreaking 1543 tome ''De revolutionibus,'' Gingerich began his 30-year crusade to restore the reputation of the Polish-born genius who first challenged the long-held notion of an earth-centric universe. With indefatigable zeal, the Harvard prof set out to locate the 600 or so extant first and second printings of Copernicus' book -- finding that most were filled with the kinds of annotations that suggested heavy use. (Take that, Mr. Koestler!) Gingerich's travels took him around the world, bringing encounters with rare-book dealers, petty Communist Party officials, larcenous bibliophiles, and at least one hostile attorney. The result is an adventure book not of the white-knuckle variety, but with a genteel, satisfying tone all its own.