Edgar Rice Burroughs is best known for his "Tarzan" and "Barsoom" novels, but they're far from the only successes he had as a writer. He also wrote another series of novels, beginning with 1914's At the Earth's Core. The novel tells the story of a man who discovers a subterranean realm known as Pellucidar, which exists at the interior of the world. There he encounters prehistoric beasts, such as dinosaurs, as well as primitive humans who have been enslaved by a race of intelligent flying reptiles known as Mahars.
Like John Carter, the arrival of the novel's protagonist, David Innes, signals the dawn of a new era. Moved by the plight of the humans -- and enraptured by the lovely Dian the Beautiful -- Innes leads them in a revolt against the Mahars and helps them to win their freedom. Along the way, he also wins the love of Dian, who'd previously rebuffed him, due to his misunderstanding the culture of the humans of Pellucidar. Unfortunately, in their attempt to return to the surface, Innes loses Dian and vows to return to find her once more, thus setting up for the novel's immediate sequel, as well as five more, one of which was published posthumously.
Arthur Conan Doyle's The Lost World predates At the Earth's Core by two years, firmly establishing the literary genre that shares its name (although several other books from the 19th century predate it and are arguably the first in the genre). Pellucidar was clearly influenced by its predecessor, but is written in a much more breezy and adventuresome style, as one would expect from the creator of Tarzan. For me, it's a much better model for pulp fantasy games than is Doyle's work, which has a vaguely "serious" tone to it, which probably explains why Gygax includes Burroughs in Appendix N and not Doyle.
As an aside, the young H.P. Lovecraft was very fond of Burroughs' stories, including the Pellucidar series, though he distanced himself from them in later life, calling Burroughs a hack. Nevertheless, some claim they hear echoes of Burroughs in the name "shoggoth," which is remarkably like that of "sagoth," the ape-like race that serves the Mahar. I'm not certain how much to make of such claims but it's interesting nonetheless.