This book is for the Deadlands role-playing game what the "Monster Manual" is for Dungeons and Dragons. It contains a few pages of information on mundane creatures like scorpions and skunks, on animal sidekicks, and on the "Explorer's Society" (sort of a London social club for gentlemen who like to hunt monsters). There are also about 4 pages or so of advice on GMing monsters. However, the vast majority of the book is devoted to describing 44 monsters for the game that (to the best of my knowledge) aren't described elsewhere: bone fiends, brain crawlers, 'gloms, pox walkers, will-o-the-wisps, etc. Each monster description is divided into two parts: a flavorful narrative description included in the player's section of the book, and a more detailed description in the GM's section of the book, which includes a description, in game terms, of the monster's abilities.
Note that unless your Deadlands character has a faithful dog or horse as a sidekick, this book is probably of more use to a Marshal (GM) than to a player. However, if you are a GM, then out of all the Deadlands books, this book is likely to be the book you consult most while preparing adventures for your players. Whether you use monsters straight out of the book, or merely read the book to give yourself ideas for creatures you invent yourself, you can expect this book to get pretty worn.
Each monster is illustrated at least once. The illustrations are all black and white, but they are well done--comparable to the art in the "Player's Guide," much better than the artwork in "Hexarcana."