Since you are going to be flooded with D20 suggestions, I figured I would go for other areas.
The Feng Shui rulebook has some excellent GMing advice about playing without maps and encouraging people to use the setting.
GURPS world books tend to have a lot of good historic and mythic information. If there is a history or mythic period you want to emulate, check for a GURPS book covering it. Things like Low Tech (for what historically was possible at various periods), Egypt, Russia, Swashbucklers, or Places of Mystery can help generate ideas for the campaign environment.
The Men At Arms series of books (Osprey Military) show various armours and uniforms. Nice if you want pictures of various period warriors.
HarnWorld has some nice material in their Pilot's Almanac for a sea faring campaign. Their Castle's book isn't quite as generic, but could still be useful.
Some books on tactics, so you can improve NPC strategies, can be a great aid. This depends on how well versed you already are in these matters.
What subject matter are you the most interested in? That would help a lot of people give you better answers.
The Feng Shui rulebook has some excellent GMing advice about playing without maps and encouraging people to use the setting.
GURPS world books tend to have a lot of good historic and mythic information. If there is a history or mythic period you want to emulate, check for a GURPS book covering it. Things like Low Tech (for what historically was possible at various periods), Egypt, Russia, Swashbucklers, or Places of Mystery can help generate ideas for the campaign environment.
The Men At Arms series of books (Osprey Military) show various armours and uniforms. Nice if you want pictures of various period warriors.
HarnWorld has some nice material in their Pilot's Almanac for a sea faring campaign. Their Castle's book isn't quite as generic, but could still be useful.
Some books on tactics, so you can improve NPC strategies, can be a great aid. This depends on how well versed you already are in these matters.
What subject matter are you the most interested in? That would help a lot of people give you better answers.