A neuronphaser.com review.
Critters, Creatures & Denizens (CCD, which happens to be a nice acronym alongside Dungeon Crawl Classics' DCC) is like the AD&D Monster Manual getting released before the AD&D Player's Handbook: it's a monster book with slightly more "advanced" rules, but it works perfectly with the existing game. To allay fears, it doesn't "change" any rules or break the game, it simply adds a little extra granularity to monster stats in the form of providing Ability Scores for all monsters, and for developing finer details for movement (running, flying, hovering). The existing stuff -- i.e. non-advanced versions of the rules -- is all there, too, so you're only getting additional information, not losing out on anything.
With that note out of the way, CCD basically replaces and expands upon most of the monster info in the Dungeon Crawl Classics RPG core rulebook. It also includes incredibly in-depth rules discussions of monsters, monster creation, and monster conversion, so the few holes in the replacements form the DCC core book (gargoyles, dragons, demons, some undead) are easy to convert to this expanded rule info if you so choose, and pulling monsters out of nearly any OSR game -- Swords & Wizardry, Castles & Crusades, and other games that have lots of really good monster books -- is a breeze. There are hundreds of monsters in here, and while a considerable number are animals, nearly every entry includes info on giant or smaller versions of animals, supernatural mutations, and other weird stuff, so that even an entry on Chickens isn't just about a chicken: there are riding chickens and war chickens! Chocobos, ride!!!
There's even a patron demon lord (Krelvax) and a mechanical faerie lord (The Gear Lord) that act as supernatural patrons or deities for a cleric, and the full rules info for such are included.
The monster entries themselves are formatted in a manner reminiscent most of the AD&D 2E Monster Manual, which is nice because it formats multiple monster entries on a single table whenever possible. For example, the Goblin entry includes 4 or 5 different goblin stats (grunt, chief, cook, etc.) and then includes a couple pages of description, combat, and ecology text that gives you great insight into how to run such creatures as more than just mini-bags of hit points trying to chew a Player Character's leg off.
I mentioned mutations. There's a whole section in the beginning about mutations, so you can pull together any existing creature (or develop a new one using the wonderful rules in this book), and then mutate it like crazy to get something horrifying and new. Or, you can mutate your player characters with minor and major mutations of great variety. Because they deserve it, that's why! And we're not talking about mutations like "becomes Spider-Man," oh no. We're talking about horrific, detrimental things that they may find benefit from, but by and large are going to send them on a death-spiral of horror, change, and being outcast from polite society (if such a thing exists in DCC!).
This book is solid through and through. Even if you don't use the expanded movement rules, don't worry about the granularity of some special abilities, and ignore mutations, you're still getting hundreds of monsters, giant versions of animals, and a few sweet patrons or familiars besides. Plus conversion rules that allow you to reappropriate any monster for the DCC game.