Harry Potter and the Prisoner of Azkaban

The third Harry Potter novel was published in Britain on 8 July 1999 and in the United States two months later, and it is the volume where Rowling starts writing about the war that came before Harry was born. The school year is built around a teaching post nobody keeps and a security arrangement the children do not trust: the Dementors are stationed at every entrance, waiting for Sirius Black, and they affect Harry more than anyone else at Hogwarts because of what he has seen.

Two new adults matter to the plot. Remus Lupin takes over Defence Against the Dark Arts and teaches Harry the Patronus Charm, the one piece of magic in the series that requires him to work with a happy memory rather than a spell; he keeps his own condition out of the syllabus. Peter Pettigrew, presumed dead since the night Harry's parents were killed, is hiding in plain sight as the family rat. The map that shows Harry every corridor of the school, and the naming of his father and his father's friends on it, are among the details that give the book its particular weight inside the series.

Prisoner of Azkaban won the 1999 Whitbread Children's Book Award and the 2000 Locus Award for Best Fantasy Novel, and it was shortlisted for the Hugo. Readers often treat it as the point where the series stopped being a school story with a mystery attached, and the time-turning machinery of its finale is the device the rest of the series builds on.