Amazon, which has a policy of not divulging exact sales figures for the Kindle and digital books, said the most recent version of the Kindle was its best-selling product ever.
"We're seeing that many of the people who are buying Kindles also own an LCD tablet," chief executive Jeff Bezos said in a statement.
Analysts have said e-readers such as the Kindle and Barnes & Noble's Nook can withstand competition from tablets among readers who prefer a device made specifically for reading.
"Customers report using their LCD tablets for games, movies and Web browsing and their Kindles for reading," Besos said.
Amazon, the world's largest online retailer, said sales of digital books on its Kindle broke a company record on Christmas.
Several Barnes & Noble locations reporting selling out of the Nook this weekend.
Amazon shares were down $1.42, or 0.8 percent, at $181.16 in morning trading on the Nasdaq Stock Market.