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.
I can live with that if I generate a PDF from web content, but I'd expect a book to have a better print version and a better quality PDF version...