Law prof Tim Wu has written a great article on JK Rowling's lawsuit against the fans who are publishing a commercial edition of the Harry Potter Lexicon website, in which he explains, in his customary clear and engaging fashion, exactly why the law is not on Rowling's side here.
Unlike a Potter film or computer game, the authors of the Lexicon encyclopedia are not simply moving Potter to another medium. Their purpose, rather, is providing a reference guide with description and discussion, rather like a very long and detailed book review. Such guides have been around forever–centuries if you count the Bible, and more recently for complex works like the writings of Jorge Borges or The Lord of the Rings. As long as a guide does not copy the original work verbatim, it falls outside the category of "adaptation." And that's why it is largely unnecessary to discuss the more complex copyright doctrine of "fair use." Rowling's rights over the guide don't exist to begin with, so we don't need to go there.