Dungeons And Dragons Online: WotC News: You'd Be Crazy Not to! -- 10 Reasons to Buy Lords of Madness

Even if the history behind aberrations isn't enough for you, there should be plenty in this book to consider. New aberrations? Check. Expanded information on current aberrations? Check. Lots of mindflayer goodness to torture your players with? Big Check. See the reasons broken down into ten easy to read points here.