What book has probably done the most "good" in all of time?

My correct answer to this conundrum will be posted in a few of days. It took me 2 years to find the answer, and I finally got it in 1990 from an ancient, gnarled little man, mundanely peering over his pince-nez as he slouched behind the counter of his used book store in Dinkytown Minneapolis, MN.