In 2012, Paizo broke the pattern they had established the previous few years of releasing a new Bestiary each fall. Instead, they released the NPC Codex, a book I raved about at the time (and still do). It quickly became one of the most useful, time-saving devices in my games. Since its release, I've been eagerly hoping for an NPC Codex 2 to cover classes outside the Core Rulebook. In 2013, however, Paizo returned to monsters with Bestiary 4, possibly indicating a new pattern of alternating years between Bestiaries and something else. That seems to have held up with last year's release of the Monster Codex. While not the NPC Codex 2 I originally hoped for, this book has quickly become something even better.
I really cannot praise the Monster Codex enough. In the year since its release, it has become one of the most used resources at my game table. It's like a Bestiary, the NPC Codex, and the Advanced Race Guide all rolled into one! The Monster Codex covers 20 of the most common monster races and provides a selection of NPCs for each, as well as several new rules options, and still more. It gives GMs a chance to take these classic monsters and add huge variety to them.
Like the NPC Codex, the Monster Codex's greatest utility is as a time saver. As with any NPC, creating a monster with class levels can be a time-consuming process, and time is something a lot of GMs find in short supply. The Monster Codex allows GMs to pull out ready-made characters in an instant, cutting out significant amounts of preparation time. Of course, it's impossible for the book to contain everything GMs could ever need, but it does include a wide variety of options, which should cover a significant chunk of those needs. I've found the book particularly useful for updating monsters from old 3rd Edition adventures. I just find the entry in the Monster Codex that is closest to the creature in the original adventure and go with that!
The book is divided into 20 chapters (one for each monster race) and 4 appendices. The monster races are in alphabetical order (and the fire and frost giant chapters are alphabetized by fire and frost, not giant as in the Bestiary). Each chapter opens with a couple pages of background information on the monster in question. This covers things like life cycle and society. The information is setting-neutral, so GMs can easily transplant it to any campaign setting or even ignore it if their chosen settings have different information. GMs using the Pathfinder Campaign Setting world of Golarion will find this information fits perfectly alongside what's been published for that setting.
After this initial information, each chapter contains a section on new rules. Similar to the race entries in the Advanced Race Guide, this section contains a variety of options specific to the monster in question. These include things like alternate racial traits and favoured class options; new feats, spells, and magic items; new archetypes and sorcerer bloodlines; and much more. Although each monster only gets two pages of these new rules, there is a remarkable amount of variety, and every new ability and item of equipment has been carefully tailored to work well with the monster. Drow get two new alchemist discoveries and some new alchemical items, fitting with their penchant for using poison and other alchemical devices. Ghouls get some creepy new feats like Bag of Bones, which lets the ghoul count as one size category smaller for the purposes of squeezing through tight spaces and a bonus to Escape Artist checks (although there appears to be a misprint here as the bonus apparently starts at +5 and increases to +4 if the ghoul has 10 or more Hit Dice; given other skill boosting feats, the bonus should probably start at +2). The feat creates the wonderfully chilling image of a creature that can dislocate its limbs and bend in all sorts of disturbing ways to fit through small spaces and continue chasing its prey.
Following the new rules, each monster race then gets a selection of NPC examples with class levels, laid out in a manner similar to the NPC Codex. The entries are in order of increasing CR, although two similar entries will be grouped together even if it takes one of them out of the CR order. For example, the kobold monster wrangler (CR 1) and kobold battle master (CR 6) both come under the header of “Kobold Riders” and are listed together before the next entry of “Kobold Tricksters”, which starts with the CR 1 kobold bomber. At first, this way of organizing the entries may seem a little confusing, but I've found that in actual use, it's incredibly efficient. I never have any trouble finding what I need. The exact number of classed characters each monster gets varies depending on the lengths of the stat blocks (with either one or two entries per page), but every monster gets six pages of entries.
Although the amount of space limits the amount of variety possible, there is still a remarkable breadth of variety present for each monster. From combat-oriented characters to spellcasters, there is a wide variety of classes for each monster. Of course, it's not possible to include an example of every class for every monster, but there is enough to cover many different situations and needs. Also, unlike the NPC Codex, the Monster Codex includes character classes from more than just the Core Rulebook, meaning there are examples of classes like cavalier and oracle. The drow poisoner (a 12th-level alchemist) is a fun one.
After the NPC examples, each chapter also contains a new monster associated with that chapter's focus monster. Sometimes, this is a variant version of the monster, such as the flind in the gnoll chapter. Flinds were part of earlier editions of Dungeons & Dragons, but have been absent from Pathfinder Bestiaries. The vampire chapter contains a more detailed vampire spawn template. The Bestiary mentions vampire spawn in its vampire entry, but instructs GMs to use a modified wight for stats. This template greatly expands GMs' options for creating vampire spawn. Other chapters include animals or creatures associated with the monsters of those chapters. The gorthek in the orc chapter, for example, is a vicious animal that orcs raise to use during warfare. The duergar chapter contains the dark spitter beetle and juggernaut beetle, giant vermin that duergar capture and ride.
The final page of each chapter contains a list of suggested encounters. These encounters provide ways that GMs can combine the various NPCs in the chapter into groups of opponents for PCs. It's a useful way for GMs to put together enemy parties quickly.
To round out the book, the Monster Codex has four appendices. The last three are different kinds of indexes (ability index, rules index, and a list of monsters by CR). The first appendix, however, is particularly useful. It contains a collection of simple class templates, one for each class in the Core Rulebook. Like other simple templates, the purpose of these is to allow GMs to modify existing monsters quickly. Each has quick rules for modifying monsters on the fly, and rebuild rules for when GMs have a little bit more time (but still not a lot). These templates are excellent for those situations when the detailed NPCs in the book don't cover your needs, but you still need something fast. They allow for huge variability in encounter options.
While there are several criteria to evaluate an RPG book with (ease of reading, unique and interesting content, balance, etc.), perhaps the most important is utility. For a book aimed at gamemasters, that utility is whether it makes the game easier to run. The Monster Codex goes way beyond expectations in this regard. Few books other than the Core Rulebook see such constant use in my games. Like the NPC Codex, it is a book that I wish existed long before it did, and it is a book that I will continue to gain use out of for years to come.