Oook -- in joke
Oook is a reference to the librarian in Terry Pratchet's Discworld series. The librarian got changed into an orangutan, and has never gotten around to changing himself back into a human: the long arms and 4 hands are so useful for filing books on the top shelves you know.
The only thing an Orangutan can say is Oook, although the other wizards don't seem to have any trouble understanding the librarian.
In joke on the in joke
The convener of the bibliophiles group, Maree Sole, is herself a librarian, although she hasn't been observed swinging through the trees or eating bananas (yet)!