A few years ago nobody criticized an author for calling his work a book if it didn't have an electronic version. We are now clearly in a transition phase [1], but it is plain that soon it will be acceptable and in fact expected to forgo the print edition when publishing "a book". A significant threat to content preservation, but that's another story.

[1] http://www.nytimes.com/2011/05/20/technology/20amazon.html