Hurok of the Stone Age(1981)
(The fourth book in the Eric Carstairs of Zanthodon series)
A novel by Lin Carter
Beneath the vast and trackless Sahara there lies a cavern so immense the whole nations could be swallowed up within it. There reside in the eternal subterranean glow the last of the Cro-magnons and their rivals the Neanderthals, lost remnants of ancient surface empires, and hordes of beasts and beings from all the ages of prehistory.
Used availability for Lin Carter's Hurok of the Stone Age
January 1981 : USA Paperback