Question: What number book in the Harry Potter series involves Snape becoming a double agent on behalf of Albus Dumbledore?
Sentence (from "Severus Snape"): A Professor at Hogwarts School of Witchcraft and Wizardry, Snape is hostile to the series' protagonist, Harry, the moment he sets foot in Snape's classroom; this hostility is rooted in Harry's resemblance to Harry's father James, who bullied and harassed Snape during their time at Hogwarts.
Relevant evidence: No