Ever since the dawn of the written word, much of publishing has been linear. A clear beginning, middle, and end has provided the framework for content over the centuries. As books developed, indexes and codices allowed people to find content without having to search from beginning to end. Digital technology has taken that to a whole new extreme.
Search engines, multiple media formats, linking, and mobile devices give people the power to dive through a universe of information without being tied to any particular source or content type. Publishers need to readjust their understanding of publishing to accommodate this. More specifically, they need to learn the technical aspects of enabling their audiences to access content in the mode of their choice. The primary way this is being accomplished today is through XML.