I can't imagine how I could ever be familiar enough with the hundreds of thousands (or is it just tens of thousands?) of fiction books released annually to say something was missing from the content. So my content answers have to be in terms of non-fiction topics not covered.
We have a child living in Taiwan and have been been reading books on the island's history. There are not enough that are well-researched, highly readable, and in English.
The more obvious answer to the OP question is that we need more backlist coming out as eBooks. Here's the latest book I've decided to read and that apparently has no eBook:
http://www.amazon.com/Lee-Teng-hui-T.../dp/1403970564
As seen in the above link, the pricing could, from a purely selfish view on my part, also use improvement. I will be asking my daughter to borrow it for me from her college library.
EDITED: I see that the linked title, while not an Amazon eBook, is available as a Nook eBook for $81.00. I wish the author and publisher the best of luck in getting others to pay this price.