"Because we can" is not a very good reason. Stamps and stickers have proven so far to work very well in identifying the owner, and if they don't _want_ to give back your book, no RFID tag will make them.
If you want to get serious, print out an empty table with four columns: "title", "lent to", "date", "signature", and everyone you give a book to has to be written down. On return they are crossed out. If you want to be fancy you can also go digital but that might be more pain than it's worth.