I found Craig Mod’s “Embracing the digital book” a helpful read in the context of a current project. It’s a nice survey of the range of issues that need to be considered when presenting a book online.
I was interested to note shortly after reading it that amazon have started publishing a table of “most highlighted passages” by kindle readers, but in practice that table’s not all that revealing. Such information needs to be much more fine grained (what’s interesting within this individual book? which of these key texts in my discipline draws most highlights? etc)