I say series, because it wouldn't do to review a specific book, because they are rather short. It takes place is a world much like Europe in the middle ages. The story follows Laurence, who is a traveling merchant that happens across a pagan goddess who has lost her followers. In a nutshell, Holo is a harvest goddess, and every year technology makes her less necessary, so she decides to move on by hitching a ride with Laurence.
The first hook of the series is the interesting concept of a normal man traveling with a magical being. It adds interest when the girl who appears young, week, and probably uneducated; is anything but. It might sound like just another 'magic girlfriend' story, but it doesn't play out that way.
The second hook is how knowledge is used. Instead of just inventing magic devices to advance the story, the tipping points come from concepts that seem basic to us, but probably would not have to someone in the middle ages. Holo the Wise Wolf, as she calls herself, is one of the few characters I've come across who the reader is not only told is wise, but actually demonstrates that.
The third hook is the interaction between the characters. I think it's an amazing example of characters, with thoughts, and feelings, who express them in the ways people do. It's never really straight forward, but people rarely are. The greatest appeal is that the characters revealed are ones the reader will want to get to know, and keep you coming back for more.