What kind of RFID reader do I need for library books?
Library systems with RFID are usually built around ISO15693 standard.
Desktop readers for this standard are relatively cheap (100$). Some libraries prefer to use UID of the tag for identification and some prefer to encode an internal unique ID in the tag (that is usually binded to the UID of the tag).
In both cases by buying a reader you won't get any spectacular results just an ID which you can usually find inscribed on the book.
For data stored on the tag, that depends on the library. A few years ago there where a few standards for encoding (like the French encoding, the Danish, etc.) Last year I believe ISO28560 was approved but I do not know which libraries adopted it.
For information stored you can have this or a combination of this:
Unique ID
EAS or AFI (security)
CollectionID
HandlingCode: can't be borrowed, special care, rare book, etc.
ISO15693 has usually 1024 bytes of user memory so any other information could be stored.
If you need a recommendation for the reader check IdTronic desktop readers.