Prisoner of Azkaban is the point at which the Harry Potter series begins to move beyond the light-weight childrens books that started it off.

It's a gentle transition, more obvious in hindsight that it would be during a first reading. It shows up in various ways, not least when we discover the cause of Snape's hatred of Harry. And while irrational, it is understandable.

But perhaps it is most obvious in the end of the book. Yes Harry pulls off yet another victory of sorts, but it's a sour victory. There's no sense of triumph this time. No clean win. No enemy vanquished.

There's also a sophistication to the story telling that wasn't there before as all the different plot elements are carefully woven together before the climax.

We are starting to see the complexities of adult life bleeding into school ever so slightly. Things aren't simply black and white any more.

The Prisoner of Azkaban may be my favorite book in the entire series. We have the depth that is missing from the early books, but without the lack of editing that shows so clearly in later ones and with the back story actually serving to drive the main plot forward rather than slowing everything down.