What is this little white sticker in books for?

Magnetic tag that sets off the alarm at the door if you try to take a book out of the library without checking it out first or stealing one from a bookseller and is most likely now an RFID tag, too.

It's no different than the security tags on clothes or jewelry at a department store.

Security Strips, Tags & Supplies - The Library Store

http://www.thelibrarystore.com/product/ad13-00475/...

3M™ RFID Tags - 3M logo

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Non-proprietary, high quality re-writable tags help libraries increase efficiency and productivity while maintaining security. Each tag contains a memory chip that can be programmed and reprogrammed with information to identify library materials.

I'd guess a magnetic tag so to set off the door alarms at a store. Perhaps an RFID tag.

The book got an owie. That's a band aid.

The book's trying to quit smoking.

Magnetic tag to reduce theft.