Representing Online Textbooks?

BWJones asks: "How does one represent online texts to best convey information to the reader as there are a number of issues related to online textbook reading and interpretation that are quite different from reading a standard textbook. We have a site dedicated to retinal vision education that went online a number of years ago and is due for a major rewrite/re-creation. This site was one of the original online textbooks and its design attempted to be of use in the early graphical navigation of the web. However, as the content has increased and will continue to do so, navigation has become more cumbersome. I am looking for suggestions to increase its usability while content increases and will entertain all ideas. I am loathe however, to make the browsing requirements too steep as there are folks from all over the world who access this site (about 30k/hits per day) and they do not always have the latest in computer technology."