Harry Potter and the Prisoner of Azkaban
This is your favorite Harry Potter book. Mine, too.
'Prisoner of Azkaban' is what really made me sit up and see these books as something much, much more than entertainment. I'm not going to say that the series reached any lofty heights of life-changing fiction. But I'm not going to not say it, either.
'Sorcerer's Stone' and 'Chamber of Secrets' were fun and a little cutesy with the darker elements absorbed fairy-tale like by the reader, noted but not really processed. Sure Harry's parents were dead, him and his friends had gone through some very dangerous situations, several kids (and a cat) had almost died. None of that mattered though because they won the House Cup! Go Gryffindor!
'Azkaban' begins to change that. There's still Quidditch and worries about getting a pass to visit Hogsmeade, but we learn more details about how his parents died, Dementors and Death Eaters, and injustice. Evil is pretty easy to do for kids, but true injustice, not just a mean potions master, presented in a way that kids will understand is quite another thing.
As much as I loved the sprawling doorstops that would follow, I like 'Azkaban' for its leanness almost as much as the story. Rowling was able to rein herself in a little towards the end, but this is the last of "typical" Harry Potter novels, more and more time would be spent away from Hogwarts after this year.
Harry Potter