First off NEVER lend a book to a friend.
If you really want them to read something you think they would enjoy, just buy them the damn book or give them your copy. Lending a book and not having it returned puts a real strain on a friendship.
Treat books correctly, especially if it belongs to someone else. I get a lot of my books at the local Goodwill stores, and I cringe when I see the way the clerks throw books into huge bins as they clear space for new titles.
If a friend lends you a book, try to read at least one or two chapters, even if it doesn't interest you in the least, do it for your friendship. Then you can honestly return it and say you tried to get into it but it just wasn't working for you, your friend will most likely understand and will appreciate the fact that you at least made the effort.
If you do lend a book to a friend don't get all "Butthurt" (as my son would say), if they don't like it. Not everyone can sit and giggle maniacally through the Flashman series. Anyway its their loss.
And because I always like to put pictures in my posts, here are some books I recently scored on Paperbackswap-