The summary of the Hero System Bestiary (6th Ed.) for the TLDR crowd: This book is the epitome of it's form, and you should own it. The longer version goes something like this:
I love monsters. Not just in a gaming sense, but in a wholistic fascination and obsessive way. I keep books on monsters, make studies of the "other" and watch plenty of B-movies. So when I say that the Hero System Bestiary is a magnificent example of what might be done with the book form, you know where I'm coming from.
First, this book opens with basically all of the tips and tricks I've devised over time as a GM, to use the bestiary to it's best effect. How all of the creatures can be generalized to fit any scenario you can envision, and then how to implement them into your game to utilize them all for the best effect. And this doesn't even begin to get into the actual monsters!
This book breaks down nearly 500 pages of monsters by genre, type, and literary source. An impressive amount of research goes into this more-than-complete book that focuses on giving the GM the tools to populate his world appropriately. It includes precisely what you need to know about the animals/monsters without drowning you in excessive detail.
This book focuses on GM usability, and is probably the best I've ever seen. There are some minor misses in the bookmarks of the electronic edition; but if you go directly by the table of contents you'll be fine. I've also mentioned this to the folks at Hero Games, and they are looking into it and will probably release an update.