Many ebooks can be read just fine on the surface area of your typical Kindle, iPad or other eReader. The reading process flows sequentially from one page to the next. No need to jump back and forth within the book or look things up in an index. Your typical novel is consumed this way, but a how-to book or a reference guide is not; with these latter examples you're often dipping in and out, jumping to and from the index and sometimes wanting to look at pages that aren't adjacent to one another.
I ran into this problem recently with a home repair project. Using my iPad I read a series of steps on how to replace a cracked porcelain soap dish on a tiled shower wall. A video was also available but when I watched it I could no longer see the numbered steps on my iPad. I needed a solution where I could could scan the numbered steps while watching the video, tying the entire process together. And if you're holding a mallet and chisel at the same time you don't have an extra hand to keep flipping screens on the iPad. What to do?
One solution is to show the video on my iPhone while the steps appear on my iPad. I actually wound up doing that, but I had to find the video link separately on my iPhone. A better solution is an eReader app that lets my devices talk to each other; touch and hold the link on my iPad screen and one of the options that pop up is, "Open link on iPhone." Just as the Camera-A and Camera-B apps let your iPad use your iPhone's camera, there's no reason an eReader app couldn't use Bluetooth to solve the problem I'm describing.
Think of it as a way of extending the surface area of your eReader device. In addition to the video feature I described above, here are a couple of other cool uses for a device-pairing feature like this:
Searches -- I'm reading a book and come across a phrase, location or some other item I want to learn more about. Today I have to leave my ebook app to do the search through my brower. I'd prefer touching the phrase on my iPad screen and having a menu option that says, "Bing search on iPhone." I never have to leave my reader app and I can click through any of the search results on my iPhone.
Index access -- Indexes are pretty clumsy in an ebook. And btw, please don't tell me indexes don't matter in ebooks. Just because I can search for any phrase I want to doesn't mean I don't want access to a well-developed index that groups all the related items, includes synonyms, etc. But thanks to the one-frame-only view in today's ebook reader apps, if I want to hop to the index I have to leave the page I'm currently reading. So how about an option where I touch the screen and one menu option is, "Display index on iPhone"? Then anytime I touch an index entry on the iPhone, the reader app on my iPad jumps to that page.
If I'm reading on my iPad I always know my iPhone is close by. That's because my iPhone is almost always clipped on my belt. The two devices are obviously capable of talking to one another, so I'm hoping that we'll soon start seeing more apps that enable this functionality. The two devices together are more powerful than either one individually.