The Book of Madness was out of print for a while, but White Wolf did a new print run of it in early '99. The book doesn't need a reprint, it needs a revision. I'll get to that.
The Book of Madness is all about the mages' strangest, wildest adversaries: The Nephandi and the Marauders, the two other forces in the Ascension War besides the Traditions and the Technocracy, along with a bunch of spirits, the demons of the Astral Umbra, Paradox spirits, and a chapter on the Umbrood in general, plus a small selections of Mythic beasts. All of it is fairly good, but none of it, except possibly the section on Paradox, was long enough for my taste. Much of the material on here was intergrated into the second edition of Mage, like the rules for Umbrood, more details about the Nephandi and Marauders, and some of the clarifications of Paradox. The lack of quantity of the material and its outdatedness are the book's two main flaws, but they're enough. Personally, I think they either need to revise and expand the Book of Madness, or, preferably, there needs to be a volume detailing both the Nephandi and the Marauders in more detail, giving some history of the two sects, and then a separate book of Umbrood, Astral demons, and Paradox spirits. But I'm just blue-skying here. This is one of those books that is all storyteller resources and nothing players would benefit from, and while not essential for running a campaign, it would definitely help if you wished to include any of the forces described within.