An article at O'Reilly Networks compares the sales of O'Reilly and Associates books at Amazon.com, Barnes and Noble, and other chains with economic indicators, such as the unemployment rate, NASDAQ average, stock prices of various companies, and consumer confidence. It's a brilliant and funny analysis told in very dry terms that spells the strong connection between specific consumer and business cyclic behavior and book sales.
I've got my own graph I can offer up from ISBN.nu, my book price comparison service.
While I'd rather not reveal sales by dollar, you can see the scope of the economy's effect on my microcosm, too.