calibre is a free and open source e-book library management application developed by users of e-books for users of e-books. It has a cornucopia of features divided into different categories.
To install Calibre open a Terminal window (Applications -> Accessories -> Terminal) and copy+paste the following lines:
sudo add-apt-repository ppa:fabricesp/ppa
sudo apt-get update
sudo apt-get install calibre
When installed you can find Calibre under Applications -> Office -> Calibre.
Some features are:
Library Management
E-book conversion
Syncing to e-book reader devices
Downloading news from the web and converting it into e-book form
Comprehensive e-book viewer
Content server for online access to your book collection
Calibre Homepage