It's hard to believe the "official" book on Riverdance hasn't arrived here yet, what with the omnipresence of the Irish dance troupe on TV (a high-profile spot on this year's Grammy Awards) and music and video bestseller charts. But now it appears there's finally an August pub date for the U.S. release of Riverdance: The Story, published in the U.K. by Andre Deutsch Ltd. and a 50,000-copy seller there. Trafalgar Square Publishing, which distributes Deutsch and bought U.S. rights to the book, has already gone back to press for an additional 10,000 copies to add to a 30,000-copy first printing. The book has also been snapped up as a selection by both the Literary Guild and Doubleday Book Club. And while the Lord of the Dance himself, Michael Flatley, has had a much-publicized split with the group, there are photos of him in this book. As of press time, Flatley and his Lord of the Dance show had only an official tour book but no other publishing deal.