Land Of Awful Shadow by Frank Frazetta

Pellucidar is perpetually lit by a central Sun, except for the "Land of Awful Shadow," beneath the moon which orbits the central Sun.  Said it before, but tis a shame we never got the two Canaveral books of Burrough's Pellucidar that were penciled-in (pun intended) to have Frazetta illos and covers, but dinnae happen