The Librarian used to be a wizard, but he was accidentally transformed into a three hundred pound orangutan. No one really knows who he used to be, and the Librarian is perfectly content with his new situation. Without any knowledge of the Librarian's previous identity, there is no known way to change him back. 
He is now only able to speak one sound "ook", and its variations, which can only be understood by those who have been around him often enough. This means that only wizards are really ever able to comprehend him now from the combinations of tone and gestures. 
He is incredibly protective of the books he looks after and woe betide anyone who dare to mar any volume. Books are usually much safer on the shelves and out of hands, but if proven trustworthy or in great need a person can be granted the privilege to borrow one.
He takes great offense to being referred to as a monkey, and will likely fly into a rage if he catches wind of it. He is an ape, thank you very much.