What makes a good monster book? Probably most importantly, you have to be able to finish it and still want to kill them. ("Good" dragons have never appealed to me just for that reason.) Beholders, crazy floating eyeball beings, have been a staple of D&D play since the early days, when they were saddled with cartoony art. They were major villains throughout the Forgotten Realms, where their magical capabilities and scheming methods made them memorable antagonists.
Goodman Games has a reputation of producing high quality, high-detail supplements and the Complete Guide to Beholders is no exception. It contains information about beholder culture, physiology, and their beliefs about the universe. It's a self-contained set of beliefs (nihilism focused on "The Void") that has attitudes about "the gods" without reference to who they are, to help you fit them into whatever campaign world that you have. There are even suggestions for how to incorporate it into superhero, espionage and horror games.
A welcome addition is a section regarding how to roleplay the beholder (don't shake your head! You know how irritating that is?!) This is something that other games should take note of. Specific advice for GMs on this level is highly needed and very effective.
There are some ideas for beholder-oriented campaigns, most of them fighting them from the outside but at least two based on being magical beings connected to beholders, or even beholders themselves! If you like play-the-monster games, it's definitely worth checking out.
I'm reviewer tilting this one up one star because I absolutely adore the roleplaying suggestion/instruction section. Surprisingly flexible and highly effective, the Complete Guide to Beholders is Well Worth The Cheap Price.