Amazon introduced the Kindle in 2007 to slow sales, which have since grown, with one report claiming the e-tailer has sold 3.3 million units of the device, to date.
Apple sold over 1 million iPads in just the first month, and international sales began today, with hundreds of thousands of sales expected in the coming weeks.
The device faces increasing pressure from rivals however, as the Apple iPad is a hit, and Sony and Barnes & Noble have both released devices that have better specs than the Kindle, at a comparable price.
CEO Jeff Bezos was asked this week if a color display version of the Kindle was forthcoming and he said there were prototypes in the lab, but none were "ready for prime-time production."