I have long been a fan of Tamora Pierce's work. The Beka Cooper series is a particular favorite. Over the series of books Beka works her way up through the "police" system in the land of Tortall. Readers will have to be able to tolerate some ambiguity. Even as a long-time reader of Pierce's books, I found myself having to "wait for it" in some of the references. In this book Beka and her police colleagues have to rush to their Lord's call on a mysterious and highly secretive mission. It's a sort of "Mission Impossible" set in Pierce's fantasy land of Tortall.
I read Mastiff using the "read for free" option on my ebook reader. That meant I had to go to the bookstore regularly to work my way through the book. I'm getting to know the staff at the local store quite well. I found myself scheduling an hour of time each and every day so that I could finish the book.
In this book Beka not only has to "save the day" but she also has to contend with several personal losses (I'm not going to be a spoiler here). Overall this in one of the most satisfying books I've read in the YA fantasy genre in a long time.