I put this together ages ago and with the three year anniversary of the campaign having just reared its ugly head, it's a good a time as any to post.
What follows is the reading list that went into the HC mental stew, I was reading or rereading (or just remembering) these books about the time the campaign (2007-2008). This list concentrates on books, but of course there are a myriad other influences such as the West Marches in particular and the general internet effervescence of the so-called OSR around that time. It's also conspicuously missing all the influential gaming materials (like Griffin Mountain).
The list below is organized in rough order of influence both by author and book. In parenthesis are my guesses of exactly what they influenced (though with all the heavy synthesizing it's even a bit of a mystery to me at this point).
Jack Vance: Lyonesse trilogy, Dying Earth, Eyes of the Overworld, Cugel's Saga, Showboat World, The Last Castle (campaign tone, themes, viewpoint of human civilization and religion, talky monsters, picaresque journey and characters etc.)
Fritz Leiber: all Nehwon stories (campaign tone and themes, direct homage in Marlankh, monsters)
John Eric Holmes: Mazes of Peril (kitchen sink, gonzo D&D)
Michael Moorcock: Elric stories (cosmology and visitors from other worlds/planes)
Clark Ashton Smith, Hyperborea cycle (well Hyperborea and some campaign backdrop)
Henryk Sienkiewicz, With Fire and Sword, The Deluge, Fire on the Steppe (eastern borderlands ambiance, both books and movies)
M.A.R. Barker, Flamesong, Man of Gold, Book of Ebon Bindings (some deep background and cosmology)
Michael Shea, Nifft the Lean, Quest for Simbilis (Vance lite)
Umberto Eco: Foucault's Pendulum, Name of the Rose (esoteric ideas, societies)
Poul Anderson: Three Hearts and Three Lions (the war between human civilization and the Fae)
Ivan Bilibin, Russian Fairy Tales (aesthetics)
Eric P. Kelly, Trumpter of Krakow
George MacDonald Fraser, Flashman series (picaros and scumbaggery)
C.V. Wedgwood, The Thirty Years War (the autarkic HRE as a model for the Overkingdom, internal religious conflict)
Stephen Turnbull, The Hussite Wars 1419-36 (backdrop of the Cantons proper)
Fernard Braudel, Civilization and Capitalism, 15th–18th Centuries (all three volumes), The Mediterranean and the Mediterranean World in the Age of Philip II (the breakdown of medieval European society and general political, economic, and cultural contours of the early Renaissance)
Steven Runciman, The Fall of Constantinople
Louise Levathes, When China Ruled the Seas (big weird ships and exploration)