The Summoning of Dragons
He then set into motion a complex plot involving setting up a secret society, the Elucidated Brethren of the Ebon Night with himself as Supreme Grand Master, and commissioned one of the Brethren, Brother Fingers (whose day job was apparently as a "capo di monty" in the Thieves' Guild, and sometime odd-job man at the University}, to go out and steal this book to order.
Wonse then discovered that a grimoire called The Summoning of Dragons does exactly what it says on the tin. In his excitement, however, he failed to read it all the way through to its slightly charred and flame-grilled last pages, written in the shaky hand of a wizard (Tubal de Malachite), who was at that point afraid for his life, and with good reason, giving the dragon a way back to reality, from which it took over Ankh-Morpork
It was given to the Librarian at the end of Guards! Guards! along with The Laws and Ordinances of the Cities Ankh and Morpork. It is now presumably hidden within the Library where no one will be able to find it.