Final answer:
Harry learned about the Room of Requirement through Neville Longbottom in 'Harry Potter and the Order of the Phoenix'.
Step-by-step explanation:
Harry learned about the Room of Requirement in the fifth book of the Harry Potter series, 'Harry Potter and the Order of the Phoenix.' In the story, Harry accidentally discovers the room when he needs a place to hide something he doesn't want anyone to find. The room appears to those who are in need, transforming itself to fulfill their desires. Harry learns about the room through Neville Longbottom, who was using it as a training room for Dumledore's Army, a secret student organization.
Learn more about Harry Potter and the Order of the Phoenix