I think a neat plugin would be to add chapter summaries to the beginning of each chapter, like some school textbooks have to help with comprehension/retention. This is a huge asset with non-fiction books and something I do manually with my epubs/mobi using OSX's built in Summerizer Service (Kindle books have the "X-Ray" function which is also really nice).
There are a number of open source summarizer software libraries in the link below which take care of text summarization. It usually spits out a list of extracted sentences with the most meaning which is great for a whole book overview, the only problem is it's difficult to map each extracted blurb to a book location/chapter which would make it a lot more useful. This plugin would need to match the summary bits to the correct epub/mobi chapter they are from and append them either into a comment or directly at the beginning of the chapter.
http://blog.mashape.com/list-of-30-s...-and-software/