This supplement for the Hell on Earth settings of Deadlands is an excellent addition to your collection. It is basically the Monster Manual for traditional D&D, but with the Deadlands flavor. As with most Deadlands books, the book is divided into three sections (Posse Territory, No Man's Land, Martial's Handbook) each providing another layer of detail into the inhabitants of the Deadlands universe.
Posse Territory includes rumors, reports, and downright lies about some of the monsters your Posse will face. This is always my favorite section of the book, as the descriptions of the beasts and events do a great job of drawing the player into the world. Before I run a game, I like to have my players at least flip through this section.
No Man's Land includes rules for creating your own animal companions. Unlike Deadlands: Rascals, Varmints, & Critters (PEG1006) for Deadlands Classic, this book doesn't just include rules for Dogs and Horses. It also has rules for cats, birds, and a few other loyal critters to tag along with your posse.
Martial's Handbook contains all the stats, rules, and truths behind the rumors found in the Posse Territory. Of course, there are also rules for creating your own monsters sure to terrify all but the most stalwart of Wasted West adventurers. That this is last in the book is supposed to indicate to your posse that this section is off limits to them.
It's the setup of these books, the meat of the world, and the fluff of the story that makes me love the Deadlands setting. Of course, this book is pretty worthless without the core book: Deadlands: Hell On Earth (PEG6000), but this is the case with any RPG.
If you're looking for something different than the newest incarnation of D20, I highly recommend checking out Deadlands. Great system.