According to this week's print edition of Business Week (March 24, 2008), e-book sales increased 59% in 2007 over 2006 based on wholesale U.S. revenues of trade book publishers. That's a significant increase, particularly when compared to the less-than-stellar increase (actually a slight decline) in p-book sales.
Also of interest are these statistics from the same page: Americans ages 20-34 spend on average 10 minutes a day reading for pleasure. Those 35-44 spend about 13 minutes; 45-54 spend about 19 minutes; 55-64 spend about 32 minutes; and 65-74 spend about 46 minutes.