I don’t know if it would be the maximum possible benefit to humanity, but I would consider sending copies of Randy Shilts’ “The Band Played On” (an early book about AIDS) to New York, San Francisco, etc. around 1965-1970. Or, better yet, any comprehensive prevention guide to HIV to be sent to every major city in the world.
Oh – and although it would clearly NOT benefit the largest possible number of people, how about sending a book about the 9/11/01 back to 1999 or so? Assuming anyone bothered to pay attention to it.