Fiction
Novels that may have inspired the campaign that players may want to read to help get in the mood. You can add comics and graphic novels to this list as appropriate, too.
Non - Fiction
Reference, history, biography, art books and other titles that may provide helpful background information for the campaign.
Movies
Inspirational movies that capture the feel of the setting.
Television
TV shows that have a similar tone to the campaign.
Music
A playlist of tunes that conveys the right feel for the game. Can be anything you like, including soundtracks to the movies listed above.
Websites
You can link to external sites with brackets.
Roleplaying Sourcebooks
I've also listed sourcebooks allowed and sourcebooks banned under @House Rules. They can be listed in either place, dependant upon what makes the most sense to you.