Early on in the series the librarian at the Unseen University is transformed into an orangutan (not a monkey).
From then on, they only say "Ook" with various inflections and changes in length. This is obviously an 'ape vocalisation' but Pratchett is known for his clever plays on words, and I wouldn't put it past him to have chose the orangutan based on the 'book'/'ook' similarity.
So is this an attempt to say "book" and a clever play on words by Pratchett or just ape vocalisation?