For $2.99, this is a steal, as it's almost 300 pages of detailed, thought-out content- and the best part of it is that those thoughts are detailed for every character. Reading the decisions behind making every race is extremely great, showing the thought process that went into each and every decision. It also contains appendixes for creating your own races using the same point-buy system- and this point-buy system is what ensures that you aren't overly punished for playing a Twig Blight over a Beholder (while the latter IS stronger, I'd say that the difference between the two is closer to a player level than between, well, the CR of the monsters). These races are designed with use as actual playable races- meaning they're designed to be paired with a class. There are also no monsters races above Medium size, for balance reasons- Giants are not all that spectacular as a result.
Many races are missing some key features, as a necessity of balance- if this turns you off, you can add them back in, but you have to be wary of making that race too powerful as a result. The author of this put in a large amount of effort to make the transition from CR15+ enemy to playable race as smooth as possible, but not everything can make it.
It is worth noting that many monsters have limitations that can make adventuring difficult, either in the short or long term. Many can't speak Common, meaning that the players will need to pick it up somehow if they plan on talking to most human NPCs. Many can't speak at all- which inhibits, among other things, spellcasting with verbal components. Many don't have hands, meaning they can't use spellcasting foci, weapons, or other tools such as wands (many without hands do have natural weapons to compensate somewhat). And many can't wear armor, which will hurt once their natural armor falls off. It is up to the player, the party, and the DM to make sure that the race you choose doesn't rely on things you can't do too badly.
This review is 4 stars because there is no 4.5/5, and there is one thing missing that stops me from giving this a perfect score: no bookmarks. If it were arranged like PDF version of the Player Handbook, with the PDF bookmarks, this would probably be five stars. While I haven't finished this yet, I already have some things in mind to use this for. Monsterous Races is a must-buy for anyone looking to play as, well, a monsterous race, as well as anyone looking to make their own (the ones in here make a good starting point to base yours on).
EDIT: Bookmarks were added, and so the one flaw I had in it is fixed! Also, if you're like me and didn't see the backgrounds at the end, be sure to look- there's also feats there too.
[5 of 5 Stars!]