They're rare books, so it's not inconceivable for someone to go through the whole game without finding one. They occasionally show up in mid- to high-level desks, and very rarely on bookshelves. I've seen two different ones while poking around, since I tend to leave no container unlooted and no crate unturned.
It's worth noting that I think both of these items were actually created by a fan as part of one of the Backer Tiers, and they may or may not be a reference to the Planescape: Torment series (Grimoire of Pestilential Thought, for example).
