How about a “library kind bomb”? Okay, I may need to explain what I mean about a “library kind bomb.” It’s a slip of paper slipped/hidden somewhere in a book that promotes the library in some way. For example, in social studies books you could include a link and the password to an appropriate database with an added note that you are always ready to help with their research needs. Or it might be a coupon for waved book fine or a free color copy. Or it might be a "if you like this book, then..." Or it might be "Smile. You've been kind bombed!" Actually kind bombing would be a fun activity for a Book Club.
Image source: http://mamascouts.blogspot.com/2012/10/guerrilla-art-kind-bombing-with-kids.html