Calibre can look a bit out of place on a gnome desktop due to the oxygen icons that come bundled with it. I've put together a partial replacement icon theme based on Tango and Humanity. Enjoy!
To use on debian-based distros, copy contents of /images/ to /usr/share/calibre/images/
For other GNU/Linux distributions, open up a terminal and use the command
to locate your install path and copy the files there.
These icons are licensed under the GPLV2+