Harry Potter and the Order of the Phoenix is the fourth book in the Harry Potter series by JK Rowling. It tells the story of Harry Potter and the evil Lord Voldemort who fought a life-and-death battle and finally foiled his evil plan to protect the magical world. This book can be purchased or read in various channels, including online bookstores, physical bookstores, libraries, e-books, etc. If you want to know some background information or plot outline before reading, you can also search for relevant information on social media or websites.