JK Rowling has updated her most recent news post with an explanation as to why she and Warner Bros. are suing a publisher and author of a Harry Potter related book.
JK's revised entry on companion books now reads:
As is now widely known, a complaint has been filed in the name of Warner Bros and myself against the publisher of a proposed Lexicon, written by Steven VanderArk. This decision was reached, on my part, with immense sadness and disappointment, and only because direct appeals for a reasonable solution failed. I never dreamed, in the light of our previous good relations – including giving the Lexicon a Fansite Award - that this situation would ever arise.
From what I understand, the proposed book is not criticism or review of Harry Potter's world, which would be entirely legitimate – neither I nor anybody connected with Harry Potter has ever tried to prevent such works being published. It is, we believe, a print version of the website, except now the information that was freely available to everybody is to become a commercial enterprise.
It is not reasonable, or legal, for anybody, fan or otherwise, to take an author's hard work, re-organize their characters and plots, and sell them for their own commercial gain. However much an individual claims to love somebody else's work, it does not become theirs to sell.
Thanks to Harry Latino for the update.
You can read the details of the lawsuit here courtesy of TLC.
UPDATE: RDR Books has posted information on the issue from their point of view.