The Internet Sacred Text Archive has this to say:
by John Uri Lloyd
Take a Victorian scifi premise, say, a trip to the center of the earth, and by the way, it’s hollow. Add a tale of a soul condemned by the Illuminati to a perilous underground quest to find the Goddess of Love (spoiler alert: spell Aphrodite backwards). Top it off with a wild magic mushroom trip. That’s Etidorhpa!
This may be the very source of the ‘adepts living in hollow earth who abduct humans’ meme, later developed by Ray Palmer, and many others. The book is larded with long passages of speculative science. The structure of the hollow earth and the effects of gravitation at various places is much better worked out than some of the ‘nonfiction’ hollow earth books (e.g. Reed or Gardner).
The journey of ‘I-am-the-man’ is a not-so-subtle allegory of spiritual progression to being a disembodied adept. Along the way he loses his youth, loses sunlight, becomes weightless, stops breathing, can hear without ears, then his heart stops, … and still he lives. Each of this steps is symbolic of a progression to a more ethereal plane of existence.
At times, the narrative recursion is three levels deep. This is an acquired taste. L. Sprague de Camp called Etidorpha ‘unreadable.’ Modern readers accustomed to consuming multiple narrative streams at the same time (i.e. channel hopping), with long recursive breaks (i.e. commercials) might do better.
Except for the titular Etidorhpa, there are no female characters. And she only appears briefly in a hallucination. Why such a small part in the book? Other genre novels, such as Atlantida and The Lost Continent, are driven by strong female characters. And once the main character is inside the hollow earth, it just halts. He doesn’t even get to meet Etidorhpa again. Whether the author ran out of steam, or the ending was only supposed to be implied, is unknown.
–J.B. Hare, Dec. 2, 2007