In the FAQ section of her newly revamped website JK Rowling addresses the issue of the Harry Potter Encyclopedia, known as The Scottish Book for longtime Potter fans. She has been working on it since Deathly Hallows came out, so this is no big shock for fans. She says:

For a long time I have been promising an encyclopedia of Harry’s world, and I have started work on this now – some of it forms the new content in Pottermore. It is likely to be a time-consuming job, but when finished I shall donate all royalties to charity.

Fans should note that this doesn’t mean it will be released any time soon as she is probably working on other projects simultaneously. It could be many years before the book is published, but it will be well worth the wait!