Hey!
I hadn't though about using Slate for reading .epub books. So, that's a really nice and useful use-case you've mentioned!
I'll definitely add this feature into the roadmap and look into adding .epub support natively into Slate, so you can read .epub files even when offline.
However, at the current stage, you can use online third part sites like THIS to open and view your .epub files from within Slate.
Also, thanks for your kind words and I hope you have a nice experience using Slate!