687 Gentleman's Guide
Unholy Sensory Magic turned out to be a dud, as Wolfe already knew most of the spells that it covered, and had extensive experience using them. On the other hand, the Gentleman's Guide to Soul Devastation was an absolute treasure of a tome, which contained not only various ways of using Unholy Magic to cause lasting soul damage, it also had ways to mend and shape the soul.
For example, the page that Wolfe was on right now, was entitled [The Skeleton Guard]. Inside, it contained a detailed description of how to modify the willpower of a skeleton to make it more loyal and powerful, while building upon various ways to enhance its undead power to make it a worthy guardian of one's household.