Harry Potter and the Prisoner of Azkaban

I think that, when thinking about the Potter novels, it's necessary to think about the first three as a trilogy in their own right, and somewhat separate from the rest of them. Several key, defining elements of the Potter mythology (such as Dementors, Patroni, and Sirius Black) are brought in in this volume, and there's a strong sense of danger and intensity throughout it that the first two books really lacked. One of the strongest in the series, for sure.