So, as is so often the case, I was looking something up in one of Tolkien's works (the extended edition of SMITH OF WOOTTON MAJOR) when I noticed something interesting in its relevance to another of his works (the Flieger/Anderson edition of OFS). And in specific to the latter's listing of fantasy authors Tolkien is known to have read.
In a passage in Tolkien's essay on SWM, he's discussing works that place Faerie underground, as opposed to his own preference, which is to associate it with The Forest.* In the course of his discussion, he alludes to such tales being "no more credible and no more interesting than Edgar Rice Burroughs['] tales dealing with a vast subterranean world" (SWM, exp.ed., page 86).
The reference here is clearly not to TARZAN or the JOHN CARTER series but to the PELLUCIDAR
series that began with AT THE EARTH'S CORE (1914), the first of six novels sharing the same setting: a Hollow Earth filled with dinosaurs, humans, and monsters of various kinds.
While Tolkien does not explicitly say he's read the E.R.B. books, I think that'd be the natural interpretation of his being able to pass judgment on them in what sounds like a well-informed personal opinion. It's easy to forget that in addition to being literary men the Inklings, especially Tolkien and Warnie Lewis, were very fond of the pulp science fiction stories of their day. I don't think I've come across Burroughs before as an author whose work Tolkien knew, but it's not surprising. So, another one to add to the list.
--soon-to-be current reading: AT THE EARTH'S CORE, as soon as I have time to download a copy. I know I started to read this once years ago but don't think I made it more than a few pages in. We'll see if I have better luck this time.
*this despite "Ides AElfscyce", which clearly places the elven-lady's abode in a subterranean world lit by a green jewel overhead, and the hidden underground cities that populate Tolkien's SILMARILLION mythos.