Apparently J.K. Rowling claims that fan-fiction, or compendium volumes are violating her copyright. Tim Wu has an excellent article here with all of the gory details, but I'm going to put my two cents in anyway.
Fan guides are a perfectly non-offensive (and, IMHO, non-copyright violating) pursuit. It's not a substitute for the actual books, and it's not an adaptation to a different media - it is, in fact, a completely new work. There are many examples of these - The Lord of the Rings has a number of intricately crafted fan guides. Even Diana Gabaldon's Outlander romance series has two companion guides (though Gabaldon is listed as the author). If Rowling wanted to ride the cash cow a little longer, she should have thought of this herself. Frankly, I am surprised she didn't, given how she wants total control over her characters, even once they've left her hands. When she announced that Dumbeldore is gay, she made it onto my "top Authors not to Like in person" list. Readers build their own sense of a character in the space where an author leaves out details. It's called engaging the imagination. Particularly for a set of children's/young adult books, there is no reason to dash these impressions that folks have developed through their reading (and often re-reading) a beloved favorite. Thank goodness they're reading at all!
Finally, you would think an author would be flattered that anyone cares to encyloped-ize (yep, just made that word up) their work. Looks like fame and fortune have given someone a big head.
This is exactly the sort of thing that makes me cranky. "Don't think that about my characters if I didn't write it!" "If there's to be an encyclopedia, I need to write it so you get it right, and don't have your imaginations running wild!" Humbug, I say.
And now I'm off to get some training on a few online survey tools. Back to the world of academic librarianship.