Harry Potter and the Prisoner of Azkaban (book #3 of the Harry Potter series) by J. K. Rowling.
This 3rd book was published in July 1999 and won the 1999 Whitbread Book Award, the Bram Stoker Award and the 2000 Locus Award. In an interesting note, this is the only novel in the series that does not feature Lord Voldemort in some form.
Harry and his best friends, Ron and Hermione, are entering their 3rd year at Hogwarts School of Witchcraft and Wizardry. In the beginning, Harry runaway from the Dursleys after getting angry with Uncle Vernon's visiting sister and blowing her up like a hot air balloon; she blows away. He has his first encounter with the Knight Bus and eventually ends up at the Leaky Cauldron, where he learns that the murderous Sirrus Black has escaped from Azkaban Prison.
On the Hogwarts Express, the train is stopped and searched by the horrible Dementors, the evil creatures who guard Azkaban - they are looking for Sirrus Black. During the course of the book, Harry learns that Sirrus Black is actually his godfather and that it was he was was responsible for the death of Harry's parents.
This was my favorite book when it was released and it still rates in my top 3 favorite Potter books. There were so many new things we learned about the world of magic and I was enchanted with all the fascinating new details. However, this is probably my least favorite of the movie adaptations.
After the release of this book, buying Harry Potter books would never be the same again. By the time Azkaban circulated the USA, Harry Potter mania was in full swing. I can still remember where I was when I waited in line to by the 4th book in the series.