We had this missive cross our desk, after first asking permission and getting a hearty lad to walk ahead carrying a red flag:
What was that old book where I don’t remember the title or the author, but the cover art was this hollowed-out human head that was unpeeling into a helix, and in the upper right there’s a planet kind-of Earthlike but not exactly, and then it’s also an apple with a huge bite taken out of it, with the background an isometric grid imposed on an outer-space shot, and in the lower right corner there’s an infinite regression of open doorframes, each with the silhouette of a stern-looking and possibly alien person in front?
PS: Love your podcast, hope to listen to it someday. Do you have one?
The answer is: you are thinking of every science fiction novel, 1970-1974. You may find some in any used book store of class III or above; best of luck with your search! Do write in if you have further questions.