The Spell Rincewind knows

pg 47

Rincewind has been expelled from Unseen University because "for a bet, he had dared to open the pages of the last remaining copy of the Creator's own grimoire, the Octavo (while the University librarian was otherwise engaged.) The spell had leapt out of the page and instantly burrowed deeply into his mind, from whence even the combined talents of the Faculty of Medicine had been unable to coax it. Precisely which one it was they were also unable to ascertain, except that it was one of the eight basic spells that were intricately woven with the very fabric of time and space itself."

In future books in the Discworld saga, it will be revealed that the Librarian had been turned into a orangatang because of a "magical accident." This accident is not going to take place in The Colour of Magic but rather in Sourcery, the second book in the series, and even then it's just mentioned in passing. The Librarian will not take on a larger role in the series until a few books later.

What's a Grimoire?

A grimoire (grim-wahr) is a textbook of magic. Such books typically include instructions on how to create magical objects like talismans and amulets, how to perform magical spells, charms and divination and also how to summon or invoke supernatural entities such as angels, spirits, and demons. In many cases the books themselves are also believed to be imbued with magical powers, though in many cultures other sacred texts that are not grimoires, such as the Bible and Qur'an, have also been believed to intrinsically have magical properties; in this manner while all books on magic could be thought of as grimoires, not all magical books could.

While the term grimoire is originally European, and many Europeans throughout history, particularly ceremonial magicians and cunning folk, have made use of grimoires, the historian Owen Davies noted that similar such books can be found all across the world, ranging from Jamaica to Sumatra, and he also noted that the first such grimoires could be found not in Europe but in the Ancient Near East.

It is most commonly believed that the term grimoire originated from the Old French word grammaire, which had initially been used to refer to all books written in Latin. By the 18th century, the term had gained its now common usage in France, and had begun to be used to refer purely to books of magic, which Owen Davies presumed was because "many of them continued to circulate in Latin manuscripts." However, the term grimoire also later developed into a figure of speech amongst the French indicating something that was hard or even impossible to understand. It was only in the 19th century, with the increasing interest in occultism amongst the British following the publication of Francis Barrett's The Magus (1801), that the term entered the English language in reference to books of magic.