There was an extreme amount of misinformation about copyrights and trademarks in this thread. A trademark does not give you absolute control over the trademarked word or phrase as was asserted by a number of posters.
As for this book, Rowling is going to lose. Unauthorized guides to fictional universes are a dime a dozen, though typically they are TV related -- such as all those unauthorized guides to Lost or BTVS which have no connection with the copyright/trademark owner and are published for the same reason any good book is, namely to make a profit.
Tim Wu did a nice job of summarizing the case against Rowling at Slate, esp. in light of Ty's failed lawsuit over a Beanie Baby collector's guide a few years ago:
http://www.slate.com/id/2181776/
Rowling will lose, and that's a good thing.