Some numbers from the Association of American Publishers from data from major book publishers comparing 2007 to 2008:
Overall sales down 2.8% (to $24.3 billion)
Trade paperbacks up, trade hardcover way down for an overall decline of 5.2% (to $8.1 billion)
Higher education textbooks up 2.7% (to $3.8 billion)
Mass market paperbacks down 3% (to $1.1 billion)
Religious books down 7.6% (to $724 million)
Audio book sales down 21% ($172 million)
eBooks up 68.4% (to $113 million)
So, while eBooks are doing well they still represent only a small fraction of total book sales.
Story and link to data here: http://www.publishers.org/main/IndustryStats/IndStats/2008/2008_Stats.htm
