J.K. Rowling said on Wednesday (July 25) that it felt "incredible" to have completed the seventh and final book on the teen-age wizard, Harry Potter. "At the moment-- it feels great, to be honest with you. It feels-- it's a really nice place to be. Yeah," Rowling said in an interview with NBC television's "Today" show, 17 years after the idea came to her as she travelled on a train from Manchester to London. The much-anticipated "Harry Potter and the Deathly Hallows" has become the fastest selling book in history, publishers said on Monday (July 24) with more than 11 million copies sold during the first 24 hours in three markets alone. Thousands of Potter fans queued outside book stores in major cities around the world over the weekend to get hold of the book, which answers the questions on every reader's lips -- 'Who dies at the end?' and in particular, 'Does Harry survive?' During an interview at Edinburgh Castle in Scotland she said, "It's a big sense of achievement. I mean I am sad, but I think (I was) sadder immediately after finishing writing. I felt devastated. For about a week I was hard to live with after finishing this book." "I think the whole thing, it was this amazing cathartic moment, the end of 17 years' work, and that was just hard to deal with for about a week," Rowling said. Rowling, who is the mother of three said finishing the book brought back memories of the last 17 years, realising she would no longer be retreating into the magical world of Harry Potter. "It brought back a lot of memories of what had been going on in my life when I started writing. When I started I wasn't in a bad place, then life had its ups and downs, so Harry has been with me through a lot. I think it was that feeling more than any other, that I wouldn't have that world to retreat into again," she said. Rowling, 41, is likely to see her fortune swell further over the coming years. She is estimated to be worth 545 million pounds (1.12 billion U.S. dollars) already, making her the first dollar-billionaire author. In addition to the books, the first five Hollywood adaptations of her Harry Potter stories have amassed around four billion (USD) at the global box office. The final film in the franchise is slated for release in 2010.