A few ideas,
Rearrange an entire shelf of books. If the shelf of books go:
133.7pla--books in between---139.7hax, reverse the order to read
139.7hax--books in between---133.7pla.
If you can do this with the entire library, go for it.
Find a book that the library does not have in their system and put it on the shelf. Go as far as putting a stamp that read "Property of the OMGPLAHAX library", the Dewey Decimal number on the spine and a clear book cover and any other markings. Since libraries have computers to check out books, the librarian will go nuts trying to check out the book for someone.
Do the same trick above but don't put any markings on the book.
Do you have access to the system that allows you to input any books in their inventory?
Pick a series of books that are checked out often. Years ago Harry Potter books were published often. Recent years we all saw the Twilight vampire books coming out. The trick is to input a book title of a popular series that doesn't exist.
When people come to look for other books in a particular series, they will be shocked to see another book in the series has been published and they will be all over the library look for the the non-existent book.
Find some company that will make a book with nothing but blank pages. Make sure the front and back of the book have writing on it. Have the front cover read "Invisible Ink: How the CIA Communicated with Spies During the Cold War". Give a fake name as the author. When people come to you about the book with no words, you simply look at a few pages and say, "damn I guess the invisible ink really does work."
While at work have a friend phone the library to have some book reserved for later pick up. Make sure they ask for the following,
Yellow River by I.P Daley
Tigers Revenge by Claude Balls
Ruptured China-man by Won Hung Lo
Brown Spots on the Wall by Wu Flung Poo
Brown Spots on the Piano Bench by Beethovens Last Movement
Lines in the Sand by Peter Dragon
50 Days in the Saddle by Major Ashburn