If I have said it once, I have said it 100 times. There is no such thing as too many monster books. This is the book you want to fill your games with all sorts of nasty beasties. The monsters are largely OGL derived and that is 100% fine by me! As with the d20 rules, True20 monsters are built like characters, so a creature that has certain powers has to be an appropriate level to have them. It means that monster building on the fly is a bit trickier till you get the hang of it. But this book provides hundreds of monsters, so that is not an issue really.
The creatures have a fantasy origin, no surprise, given True20's fantasy antecedents. The creatures here though are constrained to fantasy settings though. Dinosaurs and Dragons can attack in downtown Manhattan and vampires work well in every setting just to give a couple of examples.
Again, a must buy for any True20 fan.