The lesson here is that no matter how successful you've been, there is always more to acheive.
If you're looking for a new stream of income.. consider going back to whatever has done best for you. Rather than a new idea, JK Rowling and Scholastice don't just pump out another book.. they improve each time.
Update 4: Potter Breaks Records Across Atlantic - Forbes.com: "Harry Potter is a record breaker on both sides of the Atlantic. 'Harry Potter and the Half-Blood Prince,' the sixth of J.K. Rowling's fantasy series, sold about 9 million copies in Britain and the United States in its first 24 hours. The only book in publishing history to open nearly as well was Rowling's previous book in the series, 'Harry Potter and the Order of the Phoenix.'
'Typically, a good book will take about four to six months to go gold and very few books reach platinum in their first year of publication. 'Harry Potter and the Half-Blood Prince' achieved platinum in less than one day!' said Richard Knight, the managing director of Nielsen BookScan, which Monday reported just over 2 million sales in Britain.
British sales of 'Half-Blood Prince' were 13 percent higher than for the first 24 hours of 'Order of the Phoenix.'
In the United States, the new Harry Potter sold 6.9 million copies in its first day - averaging better than 250,000 sales per hour and easily outpacing the debut of 'Order of the Phoenix,' which came out in 2003 and sold 5 million copies.
Acknowledging that some stores quickly ran out of books two years ago, the U.S. publisher, Scholastic Children's Books, has already increased the print run for 'Half-Blood Prince' from 10.8 million copies to 13.5 million. "