Those little boxes of books on posts across America started as one man's tribute. Todd Bol built the first in 2009, from wood off his old garage door. He shaped it like a tiny schoolhouse, for his mother, June. She was a schoolteacher who tutored neighborhood kids at her dining table. He filled it with books and staked it in his yard. Neighbors wanted their own. There are now over 200,000, in 128 countries. There's probably one on your street.