The third book in the Harry Potter series:
It's time for Harry to enter his third year at Hogwarts, and it seems that it'll be just as eventful as the last two. Sirius Black, a fearsome mass-murderer, has escaped the impenetrable Azkaban and is now on the prowl. Dementors, the fortress's prison guards, have been tasked with sucking the very soul out of the escapee and any who stand in their way. Tensions are high and danger is around every corner in this exciting instalment in the Harry Potter franchise!
J.K. Rowling - Harry Potter and the Prisoner of Azkaban
Various editions available