calibre is an e-book manager. It can view, convert, edit and catalog e-books in all of the major e-book formats. It can also talk to e-book reader devices. It can go out to the internet and fetch metadata for your books. It can download newspapers and convert them into e-books for convenient reading. It is cross platform, running on Linux, Windows and OS X.
For more information, see the calibre About page
See the User Manual
Setting up a development environment for calibre
A tarball of the source code for the current calibre release.
Bug reports and feature requests should be made in the calibre bug tracker at launchpad. GitHub is only used for code hosting and pull requests.