This has to be one of the best, if not the best Monster enhancing book available to GM's. Only 1 problem, it is 3.0 and not 3.5. Beacause of this, the book has some very obvious hinderances. However, these are easily overlooked and fixed by any GM new and experienced. It just takes a bit of time. I still use this book constantly in my current 3.5 campaign.
This is not a book full of new monsters, nor is it a book full of simple templates like most other monster enhancing books are. This is a detailed book that delves into the creation of monsters, giving the DM a 'point buy' type of system for creating monsters and assigning CR's. No longer will you have to sit and wonder what a god CR is for your latest creation. Put a simple formula in on your calculater, and there is your answer.
This book covers each monster type is a fully detailed chapter. The only exception is Magical Beasts, but this can be picked up from FF's website as an enhancement for free, and you should check it out before buying the book, as it will give you an excellent idea about the book.
The first chapter details all the various supernatural, extraordinary, spell like and other such abilities of monsters. It shows you how to give creatures the use of any spell in the PHB, and most any monster ability found in the MM. It then goes on to explain how to give the monsters more flavor and description, calculating the CR, and some other basic info on moster creation.
The remaining chapters, as said above, detail each monster type. This goes into EXCELLENT detail. It gives optimum class choices for each monster type, good spells and equipment to use, feats to choose, skills, and in some cases techniques in battle. It then has a series of feats designed to that creature type, several creature type specific templates and PrC's (for each creature type). There is probably 20+ templates, and 15+ PrC's.
The book then has some new and example special abilities for that creature type, which you can add in, or simply base an ability off of, along with how much 'difficulty' it ads to the creature for calculating the CR. The chapter then ends with an example creature of the type.
You definatly get your money's worth for this book, and as long as you don't mind convertng bits over to 3.5, or if you are in a 3.0 game, then definatly a great and very handy book for any GM.