My preferred desktop environment for years has been KDE because of it’s tools and configurability. I like also XFCE, but for my day to day use I find KDE more enjoyable.
One of the applications that really makes a difference is in my opinion the best reader program available called Okular.
It is described as universal document viewer and it really is since you can open different kind of documents, like PDF, Postscript, DjVu, CHM, XPS, ePub and others.
With ePub on my Debian Jessie installation I found out that ePub documents are not associated with Okular and I was not able to open them out of the box.
All you need to get it working is to install one additional package:
apt-get install okular-extra-backends
After this you can open ePub document without any issues.