The surge in popularity of iPads, Kindles, Nooks and other digital reading devices over the holidays is showing up in the sale of e-books. 

Digital versions of the top six books outsold the print versions last week, according to “USA Today’s” Best-Selling books list.  The list, set to be published Thursday, also shows 19 of the top 50 books had higher e-book than print sales.  It’s the first time the top-50 list has had more than two titles in which the e-version outsold print.  Despite the surge, some industry watchers think it’s just a reflection of Christmas sales.  Kelly Gallagher, of the publishing research firm Bowker, says the e-book surge of the past month isn’t a “sustainable trend,” and predicts sales could flatten this year.  Still, he says, digital sales could still be twice as high as they were in 2010 when e-books sales accounted for about nine-percent of the market.