Both these books were in the same Heritage lot as the Locke book. I have a small but growing collection of books on Hollow Earth theory and the Shaver Mystery, and these two fit right in.
Bernard, Dr. Raymond (pseudonym for Walter Siegmeister). The Hollow Earth. Fieldcrest Publishing, 1964. “New Edition” hardback (I think this amounts to the second printing of the first edition, which was evidently offset, so this might qualify as the first printed edition), a Very Good+ copy in red decorated boards with a few pinhead spots of staining to rear, slight wear at head and heel, slight blunting of points, and slight wear to gold lettering, lacking the dust jacket. Barnard wrote several books promulgating various fringe and pseudoscience beliefs (vegetarianism, parthenogentic reproduction, sexual abstinence, etc.), and this book discusses how UFOs actually come from the hollow earth. He also believed there was a hollow earth opening in Brazil, and tried to start a farming colony somewhere in the general vicinity of the entrance. Kafton-Minkel, Subterranean Worlds, pages 192-216. Standish, Hollow Earth pages 277-278 (“a distillate of virtually every crackpot theory about the hollow earth that had been accumulating for a hundred years or more”). Though this had many later printings, any Fieldcrest printing seems uncommon.
Wentworth, Jim. Giants in the Earth: Ray Palmer, Oahspe and the Shaver Mystery. Palmer Publications, 1973. First edition? (no additional printings mentioned) trade paperback original, a Near Fine- copy with one tackhead-sized chip at the end of a crease to top front corner and slight wear at points, otherwise a fairly nice copy. Mishmash of Shaver Mystery, spiritualism, UFOs, Shaver’s “rock books,” and a dozen other fringe ideas, mostly taken from Palmer’s publications. Not in Kafton-Minkel or Standish.