I also use the monster books, but I also create some, and adjust existing ones. If anyone is interested, there is an excellent book(PLUG PLUG) published by Fantasy Flight Games, part of their Legends and Lairs Series, called Monster's Handbook.
It is an awesome supplement to the DM who has players who own/read all the monster books. It lays out guidelines and options to upgrade, change, redesign and totally screw up those monster encyclopedia players. The players can buy the book too, and it would do them no good, as it is the DM's imagination that guides the books possibilities, not laid out monsters. Ogres will never be the same again!!!!
It allows you to add abilities(magical or otherwise, movement, Spell-like abilities, defenses, special attacks, and special qualities, with a unique point system that adjusts the CR as you adjust the monster. You make the changes, add up the points , look up the chart, and presto new monster, new CR, and all balanced to the D&D system. I've used it several times already, and it works fantastic, and the best part, the players, who thought they knew the monster attacking them, were thrown for an awful loop. HAHA let the games begin!
er...sorry, got excited there...
*wipe drool from chin*