I helped kickstart this book. It's better than the game that gave it birth.
Apocalypse World is a game with a very simple system: you roll 2d6, and add a couple of modifiers. At 6 or below you fail, from 7-9 you succeed, though at a cost, and at 10+, you succeed big time. It's your basic postapocalyptic game. Monsterhearts takes this core system and applies it to a messy game of teenaged monsters, and blows it out of the water, because it's tradeoffs like "succeeding with a cost" that make those stories work, when they do.
The players each select a "playbook", basically a partly pregenerated character. Unlike in other games, only one player may select each playbook - you won't have a game with 5 Vampires, you will have a game with a Vampire, a Werewolf, a Mortal, a Chosen, and so on. You then mark a couple of options and you're now ready to start setting up the situation.
The situation is developed using pointed questions about relationships between and among the player characters and NPCs, who can be their fellow students in high school or their families (or substitute families in the case of Vampires or other "infectious" playbooks.)
The characters turn against each other very quickly - this is a game where PVP is part of play, but much like high school, the PVP doesn't necessarily take the form of hating each other. It could be that someone is just SO IN LOVE with someone who would prefer they jump in front of a train.
Play is relatively freeform, but that freeform is organized into "moves" that your character will inevitably end up making in their attempts to get what they want and go after their goals. What's cool is that the mechanics I mentioned above encourage you to try to get as much control of the situation before you go for what you want, but as you seek out that control, you become more compromised and other people get their hooks into you. If you make a gamble without doing that, though, you are likely to have to pay a significant cost for your success.
The key mechanic in the game is "strings". You gain strings on characters that you have emotional ties to, and those strings make your abilities, both mundane and supernatural, more effective, and give them more impact on the story. Yet at the same time, other characters will be gaining strings on you and manipulating you to pursue their goals and interests.
Advancement comes with discrete, clear steps forward in your character's effectiveness...yet on a broader scale there is a "season" advancement that only kicks in when you play for a while. This includes replacing the teenagery things you can do with more mature versions - "turn someone on" becomes "make someone feel beautiful". This kind of insight into why the characters you're playing are messed up, while still giving them a mechanical way out, is tremendously satisfying.
Do not buy this game thinking it will be a nice generic vampire game. Do not buy this game unless you have some idea why the Twilight series, as a concept, works so effectively for so many people. (You don't have to actually like Twilight to enjoy this game, since Stephanie Meyer can't actually write, just grasp what it's about.) Do not buy this game thinking it will be a Buffy-style team-up fight against monsters. (Just buy the Buffy RPG if you want that, it's pretty great.)
Do buy this game if you like developing complicated relationships between player characters. Do buy this game if you like messy situations and partial victories hard-won. Do buy this game if you want to see the Apocalypse World engine fire on all cylinders. Do buy this game if you want permission from a game system to go after what your character wants with absolutely no boundaries. Do buy this game if you want to try a game that's about your emotions as much or more than your capabilities, and links your emotions to your capabilities in an inextricable way. In short, buy this game right now, you will absolutely, positively, not regret it.
[5 of 5 Stars!]