For the first time since the Pew Internet Project began tracking e-reader use in spring 2009, ownership of the portable reading devices has reached double digits. The project found that e-reader growth has accelerated in the past six months, as tablet growth has stagnated slightly, following a steep rise in the number of users after last year’s iPad launch.

Twelve percent of U.S. adults owned a Kindle, Nook or other e-reader as of May, compared with 6 percent last November. During the same period, tablet computer ownership, including the Samsung Galaxy Tab, Motorola Xoom and iPad, rose to 8 percent from 5 percent. Approximately 3 percent of adults responding to the May survey reported having both a tablet and an e-reader.

The reason for the more rapid rise of e-reader use when compared with tablets could be due to the fact that e-readers such as the Nook Color are becoming more like tablets themselves, offering apps of all types, full-color displays and mobile operating systems such as Android. Additionally, e-readers offer such features at a much lower price point than tablets such as the iPad.

Pew also noted demographic patterns that were similar between tablet and e-reader users, although men are more likely to own tablets than women. Also, it was found that the uptake of e-readers in the past few months by parents has grown more than by non-parents. E-reader ownership among adults aged 18 to 49 grew at the most rapid pace of all age groups, while tablet ownership grew most among adults aged 18 to 29.

With Amazon potentially launching a tablet in the near future, there is great potential for tablet uptake to increase at the same rate or faster than e-reader uptake, most likely depending on the price points and features offered on such a device. Although with Barnes & Noble seeking to turn its e-reading device into a more of a tablet with each launch, Apple and other tablet makers are going to have to continue to step-up their mobile computing game to compete in this race.