Can't wait for book six?
Then try this out for size. An enthusiast all himself, W. Frederick Zimmerman has put out a book that is called the
Unofficial "Half-Blood Prince" Update:News and Speculation about Harry Potter Book Six

In this book, the author Zimmerman, pulls together everything fans know so far about the next Harry Potter book, and beyond.
In this 136 paged info-filled book, it very clearly lays out everything we know about the book so far, following all the fan rumors and speculation, debunking the rumors we know to be false, and documented with interviews J.K. Rowling has given on the subject.
In its first section, the book attempts to shed light on the new mysteries of book 6, and goes into great detail for subjects like the identity of the Half-Blood Prince, listing each of the possibilities and their pros and cons. It also includes a transcript of a very interesting chat session on Mugglenet which the author of the Update book believes was joined by J.K. Rowling herself, in which she says some very important things, lays some very important clues, and asks some very important questions we all should be asking.
In its chapters about book 7 and beyond, the book covers some possible titles that have and have not been ruled out for book 7, and also the possibility that after all the books are done, J.K. Rowling may go back and revise them all again. The book also includes many resources that fans can use to go on and learn more themselves.
A fan of the books himself, Mr. Zimmerman read all the Harry Potter books with his young daughter Kelsey, reading them aloud to her until, he says, she got too smart for him after the third book. His book is very heavily documented with quotes, so you know exactly who said these things and when they were said, making this book an important source of information you'll want to refer to over and over again.

Thanks to Wizard News for this.