Book of the Magick of Alberto Malich the Mage
A book written by Alberto Malich, the founder of Unseen University and butler to Death. It was one of several books that Albert wrote. Igneous Cutwell, a Wizard living in Sto Lat and featured in Mort, owned a copy of it. Well, an expurgated copy, anyway, with several of the more distressing pages clamped shut. Given the power of Albert as a wizard (and the fact that the distressing pages' words can be scritching irritably on a quiet night), one can assume that the original book is locked up in the Library's secure dungeons.
It contained information on the force and power of belief, and Cutwell used it for some research on the multiple timeline effect inadvertantly created by Mort. For more information, see the following article: Keli.