What's a book, though? I suppose people would at least count a traditional e-book. Does long-form fanfic count? How about a book-length website? What about one that was originally published as a book and then republished online?
I think it's just people being snooty. Bestsellers are trash, and by definition a plurality of readers are reading them. I don't think someone who reads Gladwell has any greater cognitive power than somebody who reads twitter.