Here's the previous stats and there are several caveats, such as:
The data above represent United States revenues only
The data above represent only trade eBook sales via wholesale channels. Retail numbers may be as much as double the above figures due to industry wholesale discounts.
The data above represent only data submitted from approx. 12 to 15 trade publishers
The data does not include library, educational or professional electronic sales
The numbers reflect the wholesale revenues of publishers
The definition used for reporting electronic book sales is "All books delivered electronically over the Internet OR to hand-held reading devices"
The IDPF and AAP began collecting data together starting in Q1 2006