It starts with Harry Potter enduring another summer at the Durleys', which is broken only by a news report about Sirius Black
There are a few changes at Hogwarts as Harry begins his third year. Hermione is taking a double course load, including some that are taught simultaneously. Draco Malfoy is attacked by the Buckbeak after Draco insulted him. Black is still at large, and so Dementors patrol the school. Harry faints during a Quidditch match when they approach him and falls from his broomstick. Professor Lupin teaches him the Patronus to repel Dementors. Harry receives a Firebolt broomstick during Christmas.
The Weasley twins give Harry a map. Harry uses a tunnel to sneak into Hogsmeade village where he overhears a private conversation that Black was his parents' best friend and is his godfather and legal guardian.
Lazy to write the rough storyline already... In the end, Sirius escaped on Buckbeak.
Taken from Harry Potter and the Prisoner of Azkaban by J.K. RowlingRating of this book: 7/10