Terrier by Tamora Pierce is a fantastic mystery-based fantasy novel. Or fantasy-based mystery novel? Who knows. Basically it's just really good.
I first read Terrier around the time it came out in 2006, I think. The second book hadn't been released, I thought it was a stand-alone, and I was psyched to have a new Tamora Pierce book to read. However, it'd been a long time since I'd read the first book, so while I vaguely remembered the characters and plot line, I couldn't exactly remember what had happened.
Beka Cooper is a Puppy (the training stage of becoming a Dog aka the law enforcement of Tortall). She works in the lowest areas of the city (aptly named The Lower City). Since Beka was born there and lived there for most of her childhood, she has a certain level of respect for the place. She's paired with two of the most revered Dogs, Tunstall and Goodwin. Between the three of them, they'll need to find the source of a new and semi-illegal gemstone, the murderer of seventeen adults, and the person behind the ghost that's been taking children.
This book is a bit different from Pierce's other works. It's a trilogy and each book is definitely on the bigger side for her novels (584 pages in hardcover) which I appreciate. It's also written as a journal, which takes some getting used to. I actually found myself enjoying it because Pierce/Cooper takes the time to start at the beginning of the day and not just jump to the important bits (which is what I would always do in my own journal keeping). It's also a great way to reveal Beka's feelings without awkwardly inserting them into conversation or first-person view point (even though I guess it's still in first person).
My favorite part of this book is Beka learning to make friends. She's characterized (very well, might I add) as extremely shy, so it's hard for her to make friends. And I love that Pierce can create the emotional side of a romance without actually making a romance central to the plot (or really even on screen/page at all).
Overall, I would recommend this book to fans of Pierce's other works, as well as readers looking for a good fantasy land with a mystery as the center of the plot.