Who is The Librarian from The Last Continent: A Discworld Novel and what is their importance?

A recurrent character in Terry Pratchett's Discworld novels, the Librarian has long been an orangutan. A former human, born on Moon Pond Lane, Ankh-Morpork, is changed by an accident of magic and finds life as an ape far superior. He resists all attempts to turn him back (some faculty members find it embarrassing to have an orangutan Librarian) and is in a position to remove all traces of his name from the archives. Without it, no spells can be cast on him. Years of working with books (shelving with four hands) has exposed him to a high degree of unstable magic.