E-books are available in many different formats and it really don't help users to read them on mobile devices. Thankfully Calibre can convert e-book between popular (and also not so popular) formats. I have no doubt it's the best e-book app for Ubuntu.
Calibre supports formats like: MOBI, LIT, PRC, EPUB, ODT, HTML, CBR, CBZ, RTF, TXT, PDF and LRS. So the list is long, but the most interesting feature is not only viewing those formats but also converting files between each other. Using Calivre you can also send final files to popular e-book readers.
|Calibre reader in Ubuntu. Fot. UbuCentrum.net|
sudo add-apt-repository ppa:n-muench/calibre
sudo apt-get update
sudo apt-get install calibre