Folks, thanks for the great advice. We do offer currently more than 3000 ePub ebooks (example:
http://www.lybrary.com/27913.html
However, it is true that we don't do that for our own publications. We do already have the capability to offer downloads in all available formats to our customers, but we have not yet gotten serious in converting all of our publications into ePub or other formats.
If we wanted to keep it simple for now and only choose one other format besides the PDFs we already offer, would ePub be the best choice? Or should we rather go with another format?