D&D in general: Leiber's Lankhmar, Clark Ashton Smith (Zothique, Atlantis & Averoigne in particular), Romance of the Three Kingdoms, Outlaws of the Marsh, Middle Earth, Prydain, Hyborian Age, mythology in general, Orlando Inammorato/Furioso, 13th Warrior, Red Cliff, the Harryhausen Sinbad movies and Jason & the Argonauts, Seven Samurai/Magnificent Seven, Escaflowne, Avatar: The Last Airbender, and all manner of online discussions.
Swashbuckler game: Sabatini, Dumas (Count of Monte Cristo in particular), Brust's Phoenix Guards saga, Chabon's Gentlemen of the Road, Scott Lynch's Lies of Locke Lamora, Plunkett & MacLaine.
Gothic Fantasy game: Ravenloft, Castlevania, Edgar Allan Poe, Averoigne (again), lots of Ray Bradbury, Burton's Sleepy Hollow, Brotherhood of the Wolf, some Tim Powers, the usual Stoker and Shelley and so on.
All of this is amped up by frequent diversions through Wikipedia or other sources of history. For instance, I'm finishing up MacKay's Extraordinary Popular Delusions and the Madness of Crowds, and anyone who doesn't come away with adventure ideas from a chapter on alchemy, slow poisoners, popular admiration of thieves or duels & ordeals is just not paying attention.