In addition to the first six books in the Harry Potter septology, which related books do you own? My list in order of rating:

1. The Ivory Tower And Harry Potter: Perspectives On A Literary Phenomenon (Academic essays on Harry Potter - Excellent book!)
2. Looking for God in Harry Potter (Gift from a reader, also excellent)
3. Muggles and Magic: An Unofficial Guide to J.K. Rowling and the Harry Potter Phenomenon (JKR's story, essays about the HP phenomenon, advice on writing, and a nice photo spread of UK landmarks, including the cafe where Rowling wrote the first book, and filming locations)
4. The Plot Thickens… Harry Potter Investigated by Fans for Fans (A not-bad collection of amateur analyses and fan theories)
5. The Gospel According to Harry Potter: Spirituality in the Stories of the World's Most Famous Seeker
6. Ultimate Unofficial Guide to the Mysteries of Harry Potter (Analysis of Books 1-4)
7. Unauthorized Harry Potter Book Seven News: "Half-Blood Prince" Analysis
8. The Ultimate Unofficial Harry Potter® Trivia Book : Secrets, Mysteries and Fun Facts Including Half-Blood Prince Book 6
9. God, the Devil, and Harry Potter (Haven't read yet)
10. Harry Potter and Philosophy: If Aristotle Ran Hogwarts (Haven't read yet)

I own the audio of Books 1-5 Books 1-6 as well.

One of these days, I'll write up reviews of all 10 books…