It's a good way to share books you no longer want to keep. However, some local governments are fining people for putting them up, and some people are stealing the books to sell to 2nd hand book shops - even if all they get is a few cents they're ahead.
It makes more sense than donating your books to your local library. After doing so (so those who know me can access my books), the book disappeared for over 4 months. Finally I asked, they searched and it was in the library in the State Capital, over 5 hours away. NO ONE knows me in the state capital, so the book doesn't do anyone any good there.
I'll never donate a book the library again. If they want one, they can pay for it like everyone else (I make them available for libraries cheap, although only one book was ever selected by any library).
For a long time, I offered my books in our churches annual 'Goods & Services' auction, and it was always well-bid for (often starting bidding at $20, when the retail price is only $9.99). Unfortunately, the latest auction committee decided they're no longer carrying any other items other than 'gifts' (i.e. nothing but candies, cakes or baskets of things for Valentines day).