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.Link (Thanks, Philbert!)
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.