The Inspirational Reading list from Appendix N of the 1st ed DM's Guide. de CAmp and Pratt, REG, Leiber, Vance, Merrit and Lovecraft are attributed as being the most immediate influences.
Anderson, Paul - Three Hearts and Three Lions, The High Crusade, The Broken Sword
Bellairs, John - The Face in the Frost
Brackett, Leigh
Brown, Frederic
Burroughs, Edgar Rice - "Pellucidar" series, mars series, venus series.
Carter, Lin - Worlds End series
de Camp, L. Sprague - Lest Darkness Fall, Fallible Friend, et al
de Camp & Pratt - Harold Shea series, Carnelian Cube
Derleth, August
Dunsany, Lord
Famer, P. J. - The World of Tiers series et al
Fox, Gardner - Kathar series, Kyric series et al
Howard, R. E. - Conan series
Lanier, Sterling - Heros Journey
Leiber, Fritz - Fafhrd and Gray Mouser series et al
Lovecraft, H. P
Merrit, A - Creep, Shadow, Creep, Moon Pool, Dwellers in the Mirage, et al
Moorcock, Michael - Stormbringer, STealer of Souls, Hawkmoon series (especially the first three books)
Norton, Andre
Offutt, Andrew J. editor Swords Against Darkness III
Pratt, Fletcher - Blue Star, et al
Saberhagen, Fred - Changling Earth, et al
St. Clair, Margaret - The Shadow People, Sign of the Labrys
Tolkien, J. R. R - Hobbit, the Ring Trilogy
Vance, Jack - The Eyes of the Overworld, The Dying Earth, et al
Weinbaum, Stanley
Wellman, Manly Wade
Williamson, Jack
Zelazny, Roger - Jack of Shadows, Amber series, et al.