Harry Potter and the Prisoner of Azkaban (Book 3)
Until the seventh book was released, this was my favorite book in the series. I identify it as the transition book. Books 1 and 2 are clearly meant for young readers, but books 4-7 are darker, more grown-up, and have more at stake, leaving Prisoner of Azkaban somewhere in the middle, which I think is why I love it so much. Plus: the introduction of Sirius Black.
