Harry Potter and the Chamber of Secrets.
This is, very often, the book that gets forgotten or tossed aside. I love it because it managed to be the second installment in the series and not be completely worthless, the way that most sequels usually are. Rowling came back with a second story that was just as strong as her first. I love seeing how the characters get to simply live in the universe they now know; in Sorcerer’s Stone, they were still marveling at everything. Here, they are more familiar with it and get to inhabit it comfortably. To me, that’s the mark of a great writer. Both the characters and the audience become so accustomed and comfortable with the world that’s been created, we can stretch out, walk around, and simply enjoy being at Hogwarts, in Harry’s world. Plus, it contains some of my favorite quotes of the whole series.