Harry Potter and the Prisoner of Azkaban is the third book in the series of 4. (Like I said before, there will be a total of 7) It's about 435 pages long, just a bit bigger than the first two. But this one can leave you teary eyed, hehe.

Here's a brief summary of the book:

For twelve long years, the dread fortress of Azkaban held an infamous prisoner named Sirus Black. Convinced of killing thirteen people with a single curse, he was said to be the heir apparent to the Dark Lord, Voldemort. Now he has escaped, leaving only two clues as to where he might be headed: Harry Potter's defeat of You-Know-Who was Black's downfall as well. And the Azkaban guards heard Black muttering in his sleep, "He's at Hogwarts...he's at Hogwarts." Harry Potter isn't safe, not even in the walls of his magical school, surrounded by his friends. Because on top of it all, there may well be a traitor in their midst.