Ignatius Donnelly's Atlantis: The Antediluvian World.
Erich von Daniken's Chariots of the Gods.
Morning of the Magicians.
Hamlet's Mill...
Imaginary, hypothetical history, but all based on things about which inquiring minds want to know.
What's hard to believe is that anyone thinks it shouldn't be allowed.
Only totalitarians want to ban these discussions but they have more in common with the ideas coming out of a group of friends passing around a joint and musing on the world.