You don't often get to do an experiment across the entire population of the U.S., but the Borders bankruptcy offered just that opportunity earlier this year. ICv2 notes that bookstore sales, which have been declining for years, rose 7% in the first half of 2011. Why the sharp turnaround? ICv2 attributes it to the Borders bankruptcy and the subsequent liquidation sales.
This was reflected in the September Bookscan top 20 graphic novel list, which included some older graphic novels, including Lucky in Love from Fantagraphics and the Seven Seas manga Dance in the Vampire Bund, that probably got a boost from those last-minute markdowns.
What I take away from this is that books are too expensive. E-books and online sites like Amazon have been eating away at bookstore sales for years, but apparently you can increase sales of print books in brick-and-mortar stores simply by decreasing the prices. Perhaps this is an oversimplified view of the situation, but I honestly can't think of any other reason why the trend would turn around like that. (OK, there is one: The prospect of scarcity. People who are losing their only local bookstore might be tempted to stock up, but that would only be true in a few areas.)
From everything I'm seeing, sales of e-books continued to climb during that period, which suggests a tantalizing possibility: The market as a whole, print and digital, online and brick-and-mortar, could continue to increase, if only books were cheaper. Publishers set prices based on the cost of production and the profit they want to make, but readers have their own price points—I know I do—and apparently the two don't match very well.