As I stated in my recent review of it, I really can't praise the Pathfinder RPG Monster Codex enough (so true that I'm continuing my praise here!). It's a book that has seen a ton of use in my games. As such, I was quite excited by the release of Inner Sea Monster Codex, a book that serves much the same purpose as the Monster Codex, but specifically for the Pathfinder Campaign Setting. More ready-made monster NPCs can never be a bad thing!
I have to say that it hasn't seen nearly as much use as the Monster Codex. In fact, I haven't used it at all yet, although I'm sure I will at some point. Inner Sea Monster Codex covers 10 more monstrous races, several of which, like charau-ka and gillmen, are unique to Golarion. Most of these races are not quite as common as those covered in the Monster Codex, although some, like centaurs and minotaurs, are classic monsters in the game. Although I haven't used it yet (mainly because none of the monster types within have shown up in my games since its release and it's only been out a few months), I still really like the book and look forward to using it in my games. It doesn't quite reach the accolades I have for the Monster Codex as it is much more limited in space and thus variety, but it does still have a lot to offer GMs to make their games easier.
Inner Sea Monster Codex has a similar layout to the Monster Codex. The ten monster races each get their own chapter, the chapters running in alphabetical order. Although it only has half as many monster races as the Monster Codex, it's also about a quarter the length. As such each monster race only gets about half as many pages as those in the Monster Codex do. This means there's not as much room for as many NPCs or rules options in Inner Sea Monster Codex, but despite this lack of space, the book does manage some impressive variety of characters and there are some very interesting new rules options.
Each chapter starts with two pages of background information, encounters, and new rules. It's a lot to fit into the small space, but the space is used well. The background information is, perhaps, the most disappointing, as there's so little of it and you don't really learn much new from it. Although the information is technically setting-specific, most of it is generic enough that GMs can easily adapt all the material in the book to other settings, making the book useful to all GMs regardless of what campaign worlds they use. The encounters section is a descriptive account of how PCs might run across various examples of the particular monster. It fleshes out the background information a little more, and is not a list of groups of specific NPCs like the encounters sections in the Monster Codex.
The new rules options generally take up a half to two thirds of a page and include things like new archetypes, feats, spells, and magic items. These options are tailored to fit each monster type, although many of them could be easily used by characters of other races. The cyclops chapter contains two new oracle curses (hunger and powerless prophecy) that could easily show up in other races, for example. I'm really happy to see these two new curses (plus an additional curse, site-bound, in the girtablilu chapter) as the list of available curses for oracles is very short and there have been surprisingly few expansions to the list in various supplements. My favourite new option, though, is the charger, a cavalier archetype for centaurs. This one is very much specific to centaurs only as it allows for centaurs to be their own mounts. The archetype is a great example of how to tailor a class that has abilities that don't make sense for a certain race to that race.
After the opening two pages of information, each monster then gets four NPC examples, each with a full page of stats and description. These NPCs are of various classes and cover a broad range of challenge ratings. Although this is primarily a book of antagonists, not all of the NPCs are necessarily evil. Quite a few have neutral alignments and there are even a couple with good alignments. Of course, it's always possible for GMs to tweak the fluff descriptions and give the NPCs any alignments they want (within class limits, but there are no paladins in this book).
Quite a few of the NPCs in Inner Sea Monster Codex have classes from the Advanced Class Guide, a book I'm not overly fond of, and one that I don't currently use any of the classes from. Three of the four strix NPCs, for example, have classes from that book, so I'm not sure at the moment what I'll do if I find myself needing last-minute strix characters—but this is a problem that I'm going to encounter with any book that uses Advanced Class Guide classes, and I don't, in any way, consider it a strike against this book.
One of my favourite parts of Inner Sea Monster Codex is the introduction. These opening two pages are presented as an in-game report from Yllaria Aurnosa, Council Liaison for the Promise College of Enlightened Excellence in Promise, Hermea. The report gives an overview of all ten monster races in the book from the Yllaria's biased point of view. Hermea is a region of Golarion that has not had much development, so this report gives just a little more insight into that land. I also love examples of in-world writing. They often bring across the character of a land's inhabitants far better than simple descriptive text can.
Overall, Inner Sea Monster Codex is an excellent and useful book. It doesn't quite have the utility of the Monster Codex, owing to more limited space. However, it provides GMs with ready-made NPCs that they can drop into their campaigns with only a moment's notice. Anything that cuts down on preparation time and makes GMs' lives easier is a definite success in my book.