This is a strange sort of graphic novel. I got a copy after reading a lot of positive stuff about it here and there, and also reading about the in-development roleplaying game based on it.
So, it's about medieval mice. They have a whole little medieval republic, with villages, blacksmiths and what have you. The main characters are members of the Mouse Guard, a kind of military order charged with guiding mice between towns and protecting them from predators.
It reads sort of like a fantasy story - more like that than a historical adventure - because the other creatures that the mice fight are in effect monsters for them. The story is fast-paced, dealing with a mouse traitor and our heroes discovering what is going on about that. It feels very short, very much just the first chapter of a bigger story (annoying, as the second volume is not due until the end of the year and there are four promised).
The biggest strength is the art, which is absolutely fantastic. The world of these mice is brought vividly to life, together with the scale of things they have to deal with (such as a snake in the first part). The fights look great, too. A lot of things that seem rather silly if described in words, but the art pretty much makes them work.