There are resourse books written by others concerning many great authors. There are companions for the Narnia books and LOTR books, these were not written by Lewis or Tolkien. It’s a tough call as to whether there is copyright infringement on the part of the Lexicon and I think it will be a long time before the issue is resolved. The longer it goes on the better for Rowling because it allows her time to organize her own resource book. If she completes it and publishes before the Lexicon this case becomes a moot point. Personally if I knew the publication date for the Rowling book there would be no way I would spend money on the Lexicon book. I would think Rowling's book would be a better buy because it would be more indepth.