More and more adults are turning to children's fiction - making it one of the fastest growing areas in the publishing industry, a leading bookseller said.

Increased interest in children's novels like the Harry Potter series has led to a boom in sales of 'crossover fiction', according to Waterstone's.

Pre-reservation demand for the adult edition of J.K. Rowling's next book, Harry Potter and the Half-Blood Prince, has been declared "unprecedented".

Waterstone's is predicting that more than 250,000 people will buy the adult edition of the book when it launches on July 16.

Combined sales of the adult and the children's editions should make it the fastestselling book ever, and the fastest-selling adult hardback in history.

UK sales figures released this week showed that 500,000 copies of the adult edition of the previous book, Harry Potter and the Order of the Phoenix, had brought in more than £5 million since

2003.

The biggest adult fans of the series can be found in Edinburgh, Bristol and Oxford, according to figures from Waterstone's stores. Birmingham was seventh.

The bookseller has also predicted that adult sales of Harry Potter and the Half-Blood Prince will be bigger than the total hardback sales of last year's adult bestsellers: John Grisham's The Last Juror, Martina Cole's The Graft, and Susanna Clarke's Jonathan Strange and Mr Norrell.

Other authors benefiting from the crossover boom include Philip Pullman, with the His Dark Materials trilogy, and Mark Haddon, with his The Curious Incident of the Dog in the Night-Time.