The 3rd book in the Harry Potter series follows the boy's 3rd consecutive year at the Hogwarts School that educates young wizards and witches to fight for the right cause. This time around, Harry and his two best friends - Ron and Hermione - have to learn everything they can about the Azkaban prisoner who recently broke from his prison. The wizards suspect that he might be Voldemort's biggest and oldest ally.
However, the boy doesn't know anything about this dangerous killer and only learns that he has escaped just a day before he has to get back to Hogwarts. When he arrives, Harry deals with all kinds of failures, but soon he overhears some wizards talking about Sirius Black, the escaped convict, mentioning that he used to be a close friend of Harry's mom and dad, but, one day, he betrayed them and led Voldemort, the most powerful dark wizard in the world, to their house.
Harry Potter and the Prisoner of Azkaban hit the bookshelves in 1999 and, according to Joan Rowling, she had a wonderful time writing it and finished the book in one short year. In the UK alone, the third novel in the Potter series managed to sell ~70K copies, while today the cumulative sales in England have reached a staggering number of 3 million units!
Each new book in the Harry Potter series gets more positive reviews and happy comments from the fans, and that's why Joan Rowling remains one of the best (if not the best) writers of fantasy series for children. Harry Potter and the Prisoner of Azkaban did not introduce anything new in terms of character development/twists of plot, but it was still quite an entertaining read that kept the young readers excited from page one to the very end.