According to the Association of American Publishers (pdf), e-book sales accounted for about 1.3 percent of total net sales in 2009. This is basically a rounding error, but what is interesting is how fast this sector is growing. Granted, it's from a small sales base, but the e-book category grew at about a 71 percent compound rate between 2002-2009.
The closest category was Higher Education, which increased 5 percent over the same time period. Even if the growth rates of e-books slow substantially, they look set to overtake sales from book clubs, which are declining in $ output, in just a few years. Some numbers below:
E-books sales: $ value (w/growth rate in parentheses)
2003: $19,772 (169.5%)
2004: $30,271 (53.1%)
2005: $43,832 (44.8%)
2006: $54,396 (24.1%)
2007: $67,233 (23.6%)
2008: $113,220 (68.4%)
2009: $313,167 (176.6%)
71.0% (cumulative growth rate)
See the short pdf for a table with the other categories. Additional coverage in the NYT and elsewhere.
The NYT also surveyed the current state of digital learning this weekend, in case you missed it. See here for starters. Lots of big ideas and hopes in this space, but so far I see little entrepreneurship, and what little there is, such as private online edu, seems to garner lots of animosity. It's all fascinating though.