This is the fourth book in The Iron Druid Chronicles by Kevin Hearne. These books are funny and fast paced. They're adult urban fantasy novels. If you like The Dresden Files by Jim Butcher, I would highly recommend you read this series!
The Iron Druid Chronicles aren't great literature, but they're awfully entertaining! Kevin Hearne blends celtic myth, a wry sense of humor, and a healthy dollop of the modern day into one heck of a mix.