Harry Potter and the Chamber of Secrets is the second book in the Harry Potter series. As J.K. Rowling has told us, the character of Harry Potter "really is the whole story. The whole plot is contained in Harry Potter; his past, present, and future" (source). So what would any of his books be called but Harry Potter and the Something or Other?
As for Chamber of Secrets, the Chamber is a major plot point in the story. It also sounds intriguing: if you know that someone has a secret, don't you want to hear about it? And a whole secret chamber – that's really alluring. So, if you don't want to read the novel just because it has Harry Potter in the title, maybe you'll buy it because Chamber of Secrets sounds mysterious and interesting.