J.K. Rowling's Harry Potter and the Prisoner of Azkaban
Remember when we were all waiting ages and ages for book 5 to come out, and had endless debates over which HP book was best? And I think a lot of us thought book 4 was "better" but that this one was our FAVORITE. It's where Rowling really starts to expand the wizarding world and its backstory, and also it is just awesomely action-packed (and Hermione gets to shine!). I admit that the werewolf revelation was never a shock to me--I took Latin in middle school, and we all had Latin names (I perpetually added on to mine, so by the end of 8th grade my Latin name was Claudia Olivia Lana Diana Saturnia Aventina Frumentumvir), and one of my good friends was [Latin version of her first name] with the middle name Lupa. And we used to draw little wolf faces on our notes. So if there was one word from Latin class I remembered, it was the word for wolf (and "Remus"--come on! be a little subtle!!). It doesn't matter, this book fits together so well.