I've referred to the notion of "content layering" in the past and it's a simple phrase I use to describe how a two-dimensional print product can become a richer multi-dimensional e-product. Consider a typical how-to book, for example. All the steps to accomplish a task are provided by the author and polished by the editor/publisher. If you're unfamiliar with a term you look it up in the index and hope it's defined somewhere else. If you're looking for guidance from other readers who are trying to accomplish the same task, well, you're pretty much out of luck.
The e-content world opens the door to all sorts of interesting enhancements to these and other problems. A simple pop-up on screen could show you the definition of a term, saving you the hassle of flipping back and forth to the index. That solution has existing in e-content for years, of course. The rapid growth of social networks and related technologies shed more light on what else is possible with content layering though. This post on the booktwo blog talks a bit about how the community can add more content to the work. As the post notes:
The joy of this system is that it does not rely on the publisher and the reader agreeing on what’s important information in the book - publishers can still create indexes and concordances to their work, but readers can create and share their own indexes.
It goes well beyond indexes too. Maybe you came up with an interesting workaround or shortcut to the task at hand. Great, share it with the rest of the world. Enabling this sort of rich, content layering solution could make for a fantastic e-content reader and platform. It requires extensive connectivity to the web and between all the content sources, but it's this sort of approach that could really revolutionize how content is distributed, accessed and enhanced.