Lords of Madness is the third book in the series which started with Draconomicon, and continued with Libris Mortis (the two previous books are not needed to use this book). The book describes the aberrations, one of the most intriguing, evil and alien type of monster in the D&D multiverse.
The book describes the great races, like mind flayers, beholders and aboleths (each has its own chapter with their ecology, way of life and thinking, special feats, and an example location ready to be thrown into any campaign), as well as some new aberrations (in monster-manual format). The DM has all the info needed to make his aberrations unique. No longer will the players encounter "a beholder" in the dark tunnel, but a beholder that has this and that special feat, this or that subtype, with classes, etc.
Most part of the book is for DMs, but there is also a chapter filled with goodies for players (aberration-hunters). (This also means that most players will not want to buy the book, it is enough to ask the DM to have a look at it before play...)
The book is altogether well written, and contains great ideas to make aberrations more fearful opponents, and also gives the players the opportunity to prepare against the aberration menace.
The lowpoint of the book is the monsters section which contains lot's of monsters previously published in older products, and are just updated to D&D 3.5. This is something anybody can do him/herself. More really new monsters would have been better...