The trend line certainly isn’t surprising, but it’s also important to note the caveats (from the IDPF site):
- 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
Those limitations are going to make such statistics less useful as the number of publishers selling eBooks directly increases, and as more authors take advantage of the emerging direct routes themselves, and as the lines between eBooks and other content delivered on mobile devices continue to blur.