Flummoxed by flobberworms? Fear not, the author has started work on the long-promised encyclopedia of Harry's world, with royalties to be donated to charity

Her first novel for adults is due in September, but JK Rowling isn't ready to leave behind the world of her most famous creation. The author has confirmed she is hard at work on her long-promised encyclopedia of the Harry Potter world.

In the "frequently asked questions" section on her new author site, Rowling said that "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."

Four years ago Rowling took small American publisher RDR Books to court over its plans to publish the Harry Potter Lexicon, an unauthorised A to Z of the Potter stories. Rowling called the book "wholesale theft of 17 years of my hard work", and won the case.

The news that she is finally working on an encyclopedia herself is the latest in a series of announcements from the author, from last week news that she will publish adult novel The Casual Vacancy this autumn to Saturday's newsflash that her website Pottermore is now open to all comers. While the chance to venture back between the pages of the Potter universe will be welcomed by her millions of fans – sales of ebook editions of the Potter books topped £1m in just three days when they were launched earlier this month – the author nipped hopes of an eighth Harry Potter novel in the bud.

Responding to the question "Will there ever be another Harry Potter novel?", she said: "I have always refused to say 'never' to this question, because I think it would be foolish to rule out something I might want to do in a few years' time. However, I have no immediate plans to write another Harry Potter novel, and I do think that I have rounded off Harry's story in the seven published books."

Readers on fan website Mugglenet were nonetheless rejoicing about the forthcoming encyclopedia. "Happiness x10000," wrote one fan on learning of Rowling's new project. "IT'S LIKE POTTERHEAD CHRISTMAS!" said another. "I can't even express how excited I am right now. I've been nostalgic lately, thinking back on how the series used to be, but all of this news and anticipation has reminded me that the magic is still alive. So I have one final thing to say to my fellow Potterheads, here we go again."

When Rowling published the Harry Potter fairytale spin-off The Tales of Beedle the Bard for charity in 2008, she sold 368,000 copies in just three days.