Just days after the much-anticipated arrival of Harry Potter and the Deathly Hallows, which has broken all records to become the fastest-selling book in history, JK Rowling has revealed that she has already started work on two new projects.

In an interview with USA Today, the author said: "I'm sort of writing two things at the moment - one is for children and the other is not for children." This method of composition is familiar to Rowling: "the weird thing is this is exactly how I started writing Harry, I was writing two things simultaneously for about a year before Harry took over. So one of them will oust the other in due course and I'll know that's my next thing."

Part of the phenomenal success of the Harry Potter series has been its strong crossover appeal that has led to its popularity among both adults and children. It is unclear at which age group the new book is likely to be aimed, with Rowling saying: "I don't really feel I have to prove I can write for adults, because I have already written for adults."

It is also unclear what genre the book will fall into. When asked, Rowling said: "I think that there will be some disappointment if I don't write another fantasy", but added: "I think I've done my fantasy. To go and create another fantasy universe would feel wrong, and I don't know if I'm capable of it."

Rowling admitted to being angry at leaks that meant that extracts of Harry Potter and the Deathly Hallows ended up on the internet before the official publication date: "I had been working toward that point for a long time. I did have a sense-of-humour failure when the epilogue went up."

While Rowling, whose net worth has been estimated at $1 billion (£488 million), said that she feels sadness about the end of the Harry Potter series, she went on to say that the lack of pressure on her to publish for financial reasons felt "quite uplifting", adding that she was relishing the prospect of "wandering off to a cafe with a notebook and writing."

Whatever the author publishes next is unlikely to replicate the exceptional success she has witnessed to date. "Of course I won't write anything as popular as this again. But I have truthfully known that since 1999, when the thing began to become a little bit insane. So I've had a good long time to know that, and I accept it."

Rowling expects the books to be judged by time: "When all the hype and everything else dies down, they will have to float or sink on their own merits, won't they? So in 50 years time, if people are still reading them, they deserve to be read, and if they're not, then that's OK."

However, bookstores have been upbeat about continuing sales of the Harry Potter series. Jon Howells from Waterstone's called them "very, very longterm - perennial classics", and added that they would "become the sort of book you find in every household, they have a continually refreshing audience."

That may offer little consolation to eager fans who have already finished the latest book, especially as Rowling now plans to spend "lots" of time with her family.