What number book in the Harry Potter series involves Snape becoming a double agent on behalf of Albus Dumbledore?
Severus Snape:  Snape becomes a double-agent on behalf of Albus Dumbledore and the Order of the Phoenix, and protects Harry throughout the series. Order of the Phoenix (fictional organisation): The Order of the Phoenix is a secret organisation in the "Harry Potter" series of fiction books written by J. K. Rowling. 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".
the fifth book of the series