A wave of spam clogged Kindle, the online bookstore, with material that is not only unworthy of reading but also poses threat to Amazon.com Inc’s push into self-publishing.

The best-selling Kindle e-reader accounts for around 10 per cent of the company’s revenue in 2012, Barclay’s Capital estimates. Spammers could be motivated to focus on this outlet as authors are paid 70 to 35 percent of the e-book revenue, depending on the price.

Amazon sells thousands of digital books every month, often sold at 99 cents, letting readers go through a variety of titles to find what they want.

It also offers a DVD box set called Autopilot Kindle Cash that claims to teach people how to publish 10 to 20 new Kindle books a day without writing a word.

And here comes the negative side of the online revolution that’s narrowed down the traditional publishing industry by giving authors new ways to access readers directly.

According to Albert Greco, a publishing-industry expert at Fordham University’s business school, US produced 2.8 million non-traditional books including ebooks while only 316,000 traditional books came out in 2010.

Kindle spam has been growing fast in the last six months because several online courses and, ironically, ebooks have been released that teach people how to create a Kindle book per day, according to Paul Wolfe, an Internet marketing specialist.

Smashwords, an ebook publisher and distributor, which competes with Amazon has struggled with spam but not to the extent that Amazon has done.

According to Mark Coker, Founder of Smashwords, obvious signs of spam include poorly-designed covers, the lack of an author’s name on the cover and bad formatting.

Amazon does not offer many free ebooks, while Smashwords does. So there is more of an incentive to publish lots of books via the Kindle, according to Coker.

Coker said his company has found five or six instances when free ebooks published on Smashwords were copied and republished on Amazon’s Kindle store for 99 cents each.