I just recently ran into someone making the claim that the use of "ook" to represent the sound a monkey makes is a reference to the librarian from the Unseen University of Terry Pratchett's Discworld novels. I can't find any source that actually confirms this, but I can't find any usages of the word that predates the 1986 release of The Light Fantastic either. Does anyone know one way or the other?
No, Discworld wasn't the first source to use "ook" for monkey sounds.
It's not a source I would have expected at all, but "ook" is used in the 1976 scientific publication Communication Mechanisms and Social Integration in the Black Spider Monkey, Ateles fusciceps robustus, and Related Species:
The ook-ook vocalization, although associated with grappling, is produced with many subvariants and may grade into aggressive growling or be produced in a low intensity variant, which may accompany certain aspects of sexual behavior. At no time was it inferred that ook-ook and squeak vocalizations were entirely sexually motivated or occurred entirely in a sexual context.
It was also used in What a Spot!: A Musical Farce in Three Acts (1975; a far more expected source):
(Lolita, a friendly female ape, comes loping out of underbrush, flings herself up in amiable fashion onto Rob from behind.) [...]
LOLITA. (Makes sounds to the effect of:) Ook-ook! Eeek-aak-eek!
Google Books lists the book "The behavior of Ateles geoffroyi and related species" by John Frederick Eisenberg, Robert E. Kuehn, published in 1966, as including the paragraph:
Auditory communication by [the spider monkey] appears to be well-established for some sounds, especially the whinny, ook ook, squeak, and the high-intensity agonistic sounds such as the roar and cough.
The construction of the sentence suggests that the reader would be familiar with the various descriptions, including what the difference might be between "whinny" and "ook ook". At the very least, it indicates that "ook ook" was used as the description of the vocalisation of at least one type of monkey prior to Terry Pratchett's Discworld novels.
It might be of interest that I decided to search Google Books for ook ook, rather than simply ook, in part because it seems to me to be more descriptive of a monkey's sound, but also because searching for ook on its own picks up typographical errors from words like look.