Todd Bol was simply paying homage to his mother, a schoolteacher and lover of books. He built a doll-sized schoolhouse, filled it with his mother's books and put it out for his neighbors in Hudson, Wis., as a book exchange. Today, just nine years later, more than 75,000 such Little Free Libraries" dot the globe, from San Diego to Minneapolis, and from Australia to Siberia. Why did they catch on? For starters, they promote a friendly, sharing economy. No one tracks who took what. There's no due date. No fines. You might never return a book. You might leave another instead. And, they are inherently cute. As Mr. Bol recalled, his neighbors "talked to it like it was a little puppy." This week, many bore a white ribbon in tribute to Mr. Bol, who died Oct. 18, in Minnesota at the age of 62.