A hint of what I’ve been plundering for campaign ideas:
Plus the sequel, ‘Pellucidar’.
Richard S. Shaver channels an inhabitant of the inner earth (apparently, Shaver also spent life being pursued by a demon called Max).
de Camp presents several good tales about encounters between advanced and primitive cultures.
Merritt’s tale contains my Lawful-Evil deity – The Shining One.
Farmer’s fantastic tale of characters from across earth’s history being thrown together (I’m pretty sure a Hollow Earth needs Sir Richard Burton!).
David Standish’s light but readable account of Hollow Earth history from Edmund Halley to the net’s Hollow Earth Insider.