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