From Kovid Goyal
A one stop solution to all your e-book needs. It is free, open source and cross-platform in design and works well on Linux, OS X and Windows. calibre is meant to be a complete e-library solution and thus includes library management, format conversion, news feeds to ebook conversion, as well as e-book reader sync features and an integrated e-book viewer.
I come from the Unix world, which means calibre is architected in little pieces each with its own command line interface which means that each piece of functionality is reusable not just in python programs but in any software. In fact adding complete support for converting any new ebook format to calibre requires the writing of only two converters format->html and html->format as well as a metadata reading/writing tool. All the other features of calibre will work automatically with these three converters in place. Similarly, writing a device driver requires very little work. You basically have to implement a well-defined interface (interface.py). Once you do that, your device will have full support in calibre.
Adding support for new Input/Output formats
Calibre is written primarily in Python with a few C extension modules. So you have to contribute plugins in either Python or C/C++.