I read it a while ago, but I wanted to recommend it.
Basically it tells a short novel version of the battle of Thermapoylae (aka the battle scene that inspired 300 by Frank Miller). I liked it because it shows the life of a Spartan through the eyes of a protagonist created by the author.
Just thought that it was really cool to learn about how they were trained as warriors their whole lives and how the battle was really like. If you guys liked the movie 300, you should pick this book up.