I don't think most will ever do combined purchases like that, it would be nice, but unlikely I think as there's pretty little incentive at the moment.
I think it's a bit different than the film/music markets which are going down that route because they suffer so much from digital piracy, and digital piracy exists partly because we want the freedom to move our media around our devices. While book piracy does certainly exists (usually in the form of downloadable PDFs or text files), reading for the mainstream is still more comfortable on paper.
I have a Kindle, and while it was very comfortable to actually read the text (I get headaches from reading longform text on backlit screens), it wasn't very convenient to navigate or make notes/bookmarks etc. Books/text on the whole is so much more a physical medium, while music and video is pretty much for non-interactive consumption and the only thing that really changes is the screen size/type.