The nation's largest book publishers are facing increasing pricing pressure on the digital front as the number of cheap, self-published digital titles gain popularity with readers seeking budget-minded entertainment.

Amazon.com Inc.'s top 50 digital best-seller list featured 15 books priced at $5 or less on Wednesday afternoon. Louisville businessman John Locke, for example, a part-time thriller writer whose signature series features a former CIA assassin, claimed seven of those titles, all priced at 99 cents.

"They're training their customers away from brand name authors and are instead creating visibility for self-published titles," one senior publishing executive who asked not to be identified, says of Amazon.