E-book sales are piling up fast.

In February, a couple of months after huge numbers of readers got electronic reading devices for the holidays, sales of e-books reached $90 million -- more than tripling the number from a year earlier, according to the Assn. of American Publishers.

Although that number was still smaller than sales for all paper formats combined, it outstripped any single print format -- hardcover, trade paperback or mass market paperback (think mystery novels and blocky airport fare).

Scorching e-book sales are generating another side effect, publishers said.

"Trade publishing houses cite e-books as generating fresh consumer interest in -- and new revenue streams for -- 'backlist' titles," the AAP said in a statement. Backlist titles are "books that have been in print for at least a year. Many publishers report that e-book readers who enjoy a newly released book will frequently buy an author’s full backlist."

The AAP's data comes from 84 publishing houses, including 16 that sell e-books.

Last summer, online retailer Amazon.com Inc. said sales of e-books for its Kindle reader had far eclipsed hardcover book sales, noting at the time that it had been selling e-books for only a little more over two years and had been selling paper books since 1995.

Apple's iPad has also caught on quickly as a reading device, selling more than 100 million e-books since it was introduced about a year ago.

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-- David Sarno

Image: Amazon's Kindle device. Credit: kodomut via Flickr