It's no surprise to me that this book has garnered a number of honors, including a Printz Honor Award and being part of the YALSA contingent. It's solidly written fantasy with just the right amount of real life thrown in to be appealing to those who may be thrown off by the idea of horses from the sea that ride up onto the beach and eat you.
That's a terrible description. At heart, this is probably a love story. Between a boy and a girl, but much more importantly, between horses and people. Including those horses that want to eat you. It's about loyalty as much as it is about love. It's about caring for something other than yourself, and in that, it's also about the place you live and the neighbors you have and the family that you have around you.
What I loved the most about this book was its ability to place me inside of a young girl's head, showing me what she felt about a variety of different things. This made it possible for me to learn/know/assume what she'd be thinking about any particular thing, thus avoiding the need for this kind of description and allowing the story to progress. It's really quite a perfect example of story progression.