(Wall Street Journal) — 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. As digital sales surge, publishers are casting a worried eye towards the previously scorned self-published market. Unlike five years ago, when self-published writers rarely saw their works on the same shelf as the industry’s biggest names, the low cost of digital publishing, coupled with Twitter and other social-networking tools, has enabled previously unknown writers to make a splash.