It's actually not so difficult. Getting the dependencies right can sometimes be a pain, but the Ubuntu developers have probably done that already.
Code:
sudo apt-get build-dep calibre
should install all the packages needed to build whichever version of calibre is in the repositories (we will assume you are running a recent Ubuntu release and thus the repo version is at least newer than 1.48.0).
Also install Git, for downloading the source code.
Then checkout and build the source code:
Code:
git clone git://github.com/kovidgoyal/calibre
cd calibre
python2 setup.py bootstrap
sudo python2 setup.py install