If you are at the location of the geocache you will surely notice a Little Free Library. If you are looking for a good book you can borrow one, take one, or add one.
Remember, muggles abound. People that know about the Little Free Library don't all know about the geocache. Also, the logbook that's on a shelf with the books is for the library, not the geocache. The geocache logbook is hidden with the bookmarks.