The Librarian of Unseen University is one of the most popular characters in the Discworld series to the extent where pin badges bearing the legend Librarians rule Ook are now available. The character appears or is mentioned in most books in the series.
The Librarian first appeared in the second novel of the series, The Light Fantastic, in which he was transformed into an orangutan as a side effect of a powerful magic spell. Discovering that being an orangutan has certain advantages for a librarian, he refused to be transformed back into a human, and has remained an orangutan ever since. Most of the grimoires are highly dangerous to people, and sometimes the books even consume them, but the Librarian is no person, and he no longer requires a ladder to reach those tricky high-up shelves. He retained his position as librarian despite his condition because "he's the only one who knows where all the books are" and the fact that he could screw a man's head off with his feet. The other wizards have gradually become used to the situation, to the extent that "if someone ever reported that there was an orangutan in the Library, the wizards would probably go and ask the Librarian if he'd seen it." (Night Watch).