FBReader

Calibre is the most popular application on Linux for handling ebooks. But, if you just want to read ebooks then it is too heavy a tool for that. I prefer FBReader for reading EPUB, MOBI and other ebook formats.

To install FBReader on Ubuntu:

$ sudo apt install fbreader

To associate a ebook format with FBReader look for the application named E-Book Reader in Dash.

FBReader is open source and is also available for Android from the Play Store. But the Android app seems to be pushy these days with ads, so I would not recommend it.

Tried with: FBReader 0.12.0, Android 4.4.4, Moto G and Ubuntu 14.04