I have a shelf of books that guests are welcome to take with them if they don't finish before they leave. They are either books I've read and don't care to keep in my collection, or ones that I've picked up for next to nothing at second-hand stores. Guests also leave books they've finished.
Books of short stories seem to be popular with guests, as they are easy reads while on holiday.
If I have guests who are voracious readers who are staying a week-2 weeks, I'll also let them know they can borrow any books on my "keeper" shelf, as long as they don't make off with them when they leave. Most people who like to read have respect for books and people's attachment to them and wouldn't make off with a book you asked them not to.