Is there any way to add support for epub & mobi to the default document viewer in Ubuntu?
Calibre is an overkill for me there is any other small and light software out there?
You can use FBReader. FBReader is a multi-platform e-book reader.
You can download and install from the Ubuntu Software Center, or click the image below:
Here is a free ebook tool for you.
You can easily convert your ebook from epub to pdf,epub to mobi, epub to kindle, etc.
Free Online Converter / EPUB Converter
It provides you with the best mobi to epub converting experience.