Simply put, the best Monster Manual ever.

"Premium Dungeons and Dragons Monster Manual" by Various
Published and distributed by Wizards of the Coast
ISBN = 978-0-7869-6144-0  320 pages

This is often called "Core Rulebook 3 v.3.5".  Within its pages are the closest thing to perfection that the 3.5 rulebooks have ever had.  In this Premium edition, all errata has been removed, if possible, and entries have been slightly altered to make rulings clearer.

There are seven major chapters in this volume:
     1) Monsters A to Z
     2) Animals
     3) Vermin
     4) Improving Monsters
     5) Making Monsters
     6) Monster Skills and Feats
     7) Glossary

I'm not going to bore you with long descriptions of the chapters.  But I will say that the Premium volume, with its semi-metallic cover, really stands out in my bookcases.  The art is great, as usual for WotC, and is rarely out of place.  The basics of what might be called "the ultimate D&D" are all included.  In the back of the book are two pages of Monsters sorted by Challenge Rating.  There is a really good Table of Contents, making it vary easy to find the creature you want.

Folks, if you play =any= 3.5 or D20 game, you should have a copy of this book.  The Monster Manual contains the stats for all of the basic monsters - from the Aboleth to Zombies.  Don't forget that things like cats and dogs are considered 'animals' and not monsters, and things like ants are considered 'vermin'.  If you're a GM, you gotta love the monster creating chapters.  Highly recommended.