The rasher had been placed in the pages of a reference copy of the book, which is available for general use.
Library manager Sandra Barnsley said: "I cannot believe anyone would do this, but they're not going to win.
"I apologise to all members of the Muslim community and we will do everything in our power to prevent any repeat of this incident."
A customer who was using the book for research found the bacon on Monday.
He alerted library staff, who got rid of the book immediately.
Under the terms of Islam, Muslims do not eat any pork-based products.
Sandra said: "We don't know how long the bacon had been there but obviously we were all very upset by this incident as well.
"We welcome all faiths and cultures into the library and we have never had any problems of this nature before."
"It is difficult to be vigilant at all times but I can assure all our customers we will make every effort to ensure nothing like this ever happens again."
The library has ordered a new copy of the book and is awaiting its delivery.