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.

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