This book presents the Beholder as it needs to be a nasty & rather dangerous horror waiting to take on parties of adventurers in its own environment & its own terms. This is a dungeon master book right from the get go & incorporates both the dreaded beholder & the beholder kin into a rather nasty package. There are tons of ideas here including beholder cults & all kinds of nastiness for the DM to inflict on his or her players. The eyekin are humans corrupted by beholders & are sort of available as a playable race.
Bradley K. McDevitt's artwork is among some great artwork in
The Complete Book of Beholders from Goodman Games
The Complete Book of Beholders from Goodman Games came out right in the middle of the D20 boom & its still a great game book to use to add beholders into a game campaign. Here's where the dice hit the table. The Complete Guide to Beholders could be easily used as a launch point for a full scale beholder invasion of a campaign.
The Eyekin warriors planarly gate into the campaign world & begin to fry everything. Those that survive are going to be the party of adventurers. The PC funnel is now complete. Step two is to introduce the various beholder generals & warlords leading the army. The PC's find out about the plans of the beholders through a dungeon or adventure location & then begin to incorporate their own resistance. And the campaign continues with the DM trotting out various beholder types & horrors featured in the book. This same plan could be used with Mutant Crawl Classics as well to really change events up in a Mutant Crawl Classics game.
In terms of the Castles & Crusades rpg retroclone the switch over with the The Complete Book of Beholders from Goodman Games isn't all that of a power stat struggle. The book is very low key when it comes to working up the stats over to a campaign. The Complete Book of Beholders from Goodman Games is also a perfect add over for a game of the Amazing Adventures rpg.
There's a great article on using eyekin as PC's on Geek Native right over here.