Question: What number book in the Harry Potter series involves Snape becoming a double agent on behalf of Albus Dumbledore?
Sentence (from "Order of the Phoenix (fictional organisation)"): Founded by Albus Dumbledore to fight Lord Voldemort and his followers, the Death Eaters, the Order lends its name to the fifth book of the series, "Harry Potter and the Order of the Phoenix".
Relevant evidence: Yes