There are a few around here. I find sodden books in the woods. I imagine the homeless use them to start fires. Probably a source of toilet paper in SF.
The protocol is to put in as many books as you take out. It works on the honor system. I use them often and they are apparently being used because I'm always seeing new titles in them. It's also fun to see some books sit there forever, with no takers. That Michelle Obama book is a good example of a "forever" book - it's been sitting at the bottom of the mini-library by the Still River walkway in Brookfield for almost two years now. Books I deposit there are usually gone within a few days and I always find some good books to take out.
Never had a problem with "soggy" books. These mini-libraries usually have watertight doors and shingled roofs like you would find on a house. The craftsmen who build these things put a lot of effort in to them.
It's good to live in neighborhoods where you can actually have nice things.