So, since finding Tolkien's passing reference to Burroughs' Pellucidar, and his (already well known but forgotten by me) familiarity with Burroughs work in general, esp. Tarzan, I've been reading a copy of Lupoff's book* to get a sense of the context within which he quotes Tolkien.
First off, I have to say Lupoff's is an enjoyable book and I've learned a lot from it. It's not a biography but an overview of Burroughs' literary career, looking at each of his seventy-odd book. Since Burroughs wrote multiple series, switching back and forth between the latest entries in each, it's particularly helpful that Lupoff doesn't follow a strict chronological sequence but groups books belonging to various series and sub-series together and discusses them in said groupings. It's also good that, while obviously a great admirer of E.R.B.'s work, Lupoff also think Burroughs wrote too many books and takes it as his task to highlight what he thinks are Burroughs' best works and why., and which are only for completists.** I tried to do something of the same with Dunsany in my dissertation, and think he does a good job with this difficult task. He's also not shy about ranking some ERB books as abject failures, either from Burroughs trying to write outside his comfort zone or by-the-numbers later entries in series that had outstayed their welcome.***
He also includes a chapter on likely influences on Burroughs and another on Burroughs' (massive) influence on others, the most notable of whom he considers to be Rbt E. Howard. (who I'd say has somehow come to eclipse even Burroughs himself in recent decades). And in addition to revisions and updating by Lupoff himself, there's also an extensive chapter, added by Burroughs scholar Phillip Burger, covering events of the forty years between the editions of Lupoff's own book.****
To my surprise, the idea that Tolkien was influenced by Edgar Rice Burroughs comes not from Lupoff but from Tolkien's friend Roger Lancelyn Green. The three relevant paragraphs in Lupoff's book read as follow:
A final possible descendant of John Carter, suggested
by Roger Lancelyn Green in Into Other Worlds,
appears in The Two Towers by J. R. R. Tolkien.
The Two Towers is part of Professor Tolkien's great
trilogy The Lord of the Rings (George Allen and
Unwin, 1954, 1955, 1956). Green suggests that
"Shelob in The Lord of the Rings is so like the
Siths of the Barsoomian caves that an unconscious
borrowing seems probable."
I personally find Shelob, a huge anthropophagous
spider who guards a tunnel, more closely analogous
to the apts which guard the carrion caves of Okar
in The Warlord of Mars. Queries as to the possibly
attribution, Professor Tolkien offers this gracious
but not very helpful reply:
Source hunting is a great entertainment but I do not
myself think it is particularly useful. I did read many
of Edgar Rice Burroughs' earlier works, but I developed
a dislike for his Tarzan even greater than my distaste
for spiders. Spiders I had met long before Burroughs
began to write, and I do not think he is in any way
responsible for Shelob. At any rate I retain no memory
of the Siths or the Apts.
(Lupoff, rev. ed. pages 200 [1st 2 paragraphs] & 201 [3rd paragraph]).
I have to confess to not having read WARLORDS OF MARS (1919), third book in the original Barsoom trilogy; from a quick dip it sounds as if the SITH is in fact a giant wingless hornet, while the APT sounds more like the Lovecraftian ghoph-keh.
In any case, Lupoff first devotes a paragraph to summarizing Green's point, then follows with a second paragraph advancing a suggestion of his own, then concludes with the third paragraph by Tolkien himself denying the connection, in words rather similar to his parallel critism of the Lord Peter Wimsey series: perhaps he simply had low tolerance for series that went on and on.
The irony in this is that Green, who knew Tolkien (JRRT had been his advisor for his Thesis and may have modeled one of the characters in THE NOTION CLUB PAPERS on RLG) and could have just asked him directly about any possible borrowing, puts forward a suggestion just on his own cognizance, so to speak, while the American Lupoff writes to the overseas author seeking confirmation.
*MASTER OF ADVENTURE: THE WORLDS OF EDGAR RICE BURROUGHS by Richard A. Lupoff (rev. and exp. ed. 2005; orig publ. as EDGAR RICE BURROUGHS: MASTER OF ADVENTURE in 1965)
**particularly helpful is the chapter titled "A Basic Burroughs Library" (p.202-213), in which Lupoff says that if you're only going to read one book by Burroughs, it shd be TARZAN OF THE APES, the first Tarzan book. But if you're going to read two, then A PRINCESS OF MARS shd be right there in second place. And if you're going on to three he'd suggest THE WAR CHIEF, a lesser known western. From there he expands this to a half-dozen, adding THE MUCKER (another non-series book), THE MOON MAID (as among his best science fiction), and TARZAN AND THE ANT MEN (as the best of the later Tarzan books). If going for a full dozen, he recommends AT THE EARTH'S CORE (the first Pellucidar book), TALES OF THREE PLANETS, THE GIRL FROM HOLLYWOOD (simply because it's wholly atypical), THE LAND THAT TIME FORGOT (Burroughs dipping into Conan-Doyle territory), and the other two books in the original Barsoom trilogy: THE GODS OF MARS and THE WARLORD OF MARS.
***It's good to remember that Burroughs, like H. Rider Haggard and Earl Stanley Gardner, wrote quickly: each book took an average of forty-nine days to write, according to Lupoff (p. 53), or about seven weeks per book.
****have to admit that I'm amused by the fact that this book has a Foreword
(by Michael Moorcock), an Introduction
(by Lupoff, to the new updated and expanded edition), a Preface
(by a fulsome Burroughs bibliographer named Henry Hardy Heins, who's writing for fellow enthusiasts), and another Introduction
(again by Lupoff, to the original 1965 edition).