MYSTERIES FROM FORGOTTEN WORLDS by Charles Berlitz

Here's Berlitz' take on pre-Ice Age super civilizations, written in his usual plodding style. Borrows heavily from Charles Hapgood and his Maps of the Ancient Sea Kings, another foundational text for the 70s boom of paranormal paperbacks. Available to read and download at archive dot org.

Dell Publishing, 1973

MYSTERIES OF TIME AND SPACE by Brad Steiger

Prolific author Steiger delivers more of those all-powerful ultra-terrestrials and their enigmatic calling cards, including Momo the Missouri Monster and the Men in Black! 

Dell Publishing, 1974

COSMIC DEBRIS: Occult Encyclopedias

Whither Zolar? A pull-out ad from The Lost Continent of Mu by James Churchward. Courtesy the Paperback Library, 1972.

MAPS OF THE UNKNOWN: The Great Pacific Gas Belts

From The Lost Continent of Mu by James Churchward. Courtesy the Paperback Library, 1972 (original pub. 1931).

THIS HOLLOW EARTH by Eric Norman

Another cheap paperback that could have used some illustrations. It's also missing a table of contents! Available to read and download at archive dot org.

Chapters:

1.  The Weird World Inside the Hollow Earth (5)

2.  Mystery of the Hole in the Poles (25)

3.  The Secrets of Shamballah (45)

4.  Doc Anderson - Pyramids, Tunnels, and Tibet (63)

5.  Subterranean Tunnels and Treasure Vaults (85)

6.  The Coming Race and Vril Power (105)

7.  Genesis of the Vril-ya (117)

8.  Adolph Hitler: Messiah of Hollow Earth Lore (129)

9.  Exodus: Demons and the Shaver Mystery (145)

10. Believers in the Hollow Earth (161)

11. Cook and Peary: The North Pole Controversy (179)

12. Tamil and the Cave Masters (197)

Lancer Books, 1972

ATLANTIS RISING by Brad Steiger

Steiger zooms about on a whirlwind tour gathering all manner of weird things into the Atlantis Mythos, including the Shaver Mystery, Hollow Earth, and that old scamp Springheel Jack! Some wild fun, but like many of the cheaper paperbacks it would have benefited from some illustrations and/or maps.

And a lovely painted cover from Sphere Books, 1976.

Dell Publishing, 1973

COSMIC DEBRIS: Bermuda Triangle Clipping

Clipping from the Portland Oregonian, found tucked away inside The Bermuda Triangle Mystery - Solved by Lawrence David Kusche. Courtesy Warner Books, 1975.

MAPS OF THE UNKNOWN: Antarctica Exposed

From Mysteries From Forgotten Worlds by Charles Berlitz, following Charles Hapgood and his Maps of the Ancient Sea Kings. Courtesy Dell Publishing, 1973.

THE FLYING SAUCER STORY by Brinsley Le Poer Trench

Ace Books, 1966

UFOs AND THEIR MISSION IMPOSSIBLE by Clifford Wilson

Signet Books, 1975