"A reference to 'Joad' in a letter written in 1948 suggests
that he knew John Steinbeck's Grapes of Wrath (1939)."
--READER'S GUIDE, page 818.
Now, on the surface this is not as unlikely as it looks. After all, it's no more implausible that Tolkien wd know Steinbeck's works than that he wd be familiar enough with Sinclair Lewis to suggest that the word 'hobbit' owed something to the latter's BABBITT (cf. MR. BAGGINS page 59, Nt 6). And there's Clyde Kilby's observation that he "was pleasantly surprised at the familiarity he showed with American literature, especially that of Mark Twain" (TOLKIEN & THE SILMARILLION, pages 30-31). By contrast, fellow Inkling Warnie Lewis knew so little about American lit. that late in life, when sent a postcard of Hawthorne's cottage by Kilby, Barfield, and Lawlor, he cheerfully confessed to never having read Hawthorne "but I like the look of his house" (Lawlor, MEMORIES & REFLECTIONS, page 21).*
But it turns out, the only evidence we have to even suggest it -- Tolkien's mention of the name 'Joad' -- turns out to be a false clue. Rather than a fictional character in an American novel about the Great Depression and Dustbowl (and old Henry Fonda movie), it's far more likely that the 'Joad' Tolkien is referring to is the British philosopher, writer, and radio personality C. E. M. Joad, nicknamed 'Joad of Joad Hall' for his resemblance (in personality) to THE WIND IN THE WILLOWS' Mr. Toad. Tolkien had already referred to Prof. Joad in two earlier letters of October 1943 [LETTERS p. 63); indeed, apparently Joad attended an Inklings meeting on October 26th, 1943. That the same person is intended in Tolkien's 1948 letter is shown by a specific detail: when Tolkien writes that an artist has drawn Farmer Giles "to look like little Joad at the end of a third degree by railway officials" [LETTERS p. 131], he's clearly alluding to the scandal that cost Joad his radio show: he happened to casually mention one day, on the air, that he never paid for rail tickets but simply slipped on, rode for free, and had always gotten away with it for years and years. There was an official outcry, and much tsk-tsking about setting bad examples for the young, and the general upshot was that Joad was driven from public life, suffering a debilitating heart attack shortly afterwards and dying a few years later. A sad end for someone who helped popularize philosophy in the same way Lewis popularized Church of England theology at much the same time.
And, it turns out Wayne & Christina caught the slip almost immediately, the sharp-eyed David Doughan having spotted the slip as soon as the book came out and their correction being posted among their scrupulous online errata (the whole of which makes for a fascinating read all by itself).
Of course, such a tiny slip does not detract from the excellence of their overall essay ("Reading"), which if published by itself wd probably have attracted attention as a major piece of JRRT, pulling together a vast amount of information into a concise space. There's just no avoiding the fact that, when we draw inferences based on sketchy information, sometimes a reasonable guess will turn out to have been wrong. In a sense, a book like THE HISTORY OF THE HOBBIT or their COMPANION & GUIDE is an ongoing project -- the road really does go ever on, so to speak.
And, since I enjoyed reading this piece so much, I'd like to add a few tidbits:
(1) not only did Tolkien know some Twain (who after all was a celebrity in England as well, having visited Oxford to receive an honorary degree in 1907, just a few years before Tolkien arrived there as an undergraduate), but
(2) he read at least some of GKC's FATHER BROWN stories, since Kilby -- who after all included both Chesterton & Tolkien among the 'Seven' authors of his Wade Collection -- noted of that "He did not care for the detective stories of G. K. Chesterton" -- TOLKIEN & THE SILMARILLION, page 26) [at least as of 1966].
(3) Since Tolkien stated that the title of SMITH OF WOOTTON MAJOR was "intended to suggest an early Woodhouse" [LETTERS p. 370], it seems likely that he read at least some P. G. Wodehouse.
current reading: many bits and pieces.
*CSL had a good many books by American authors in his library, but a number of these (e.g., Cabell) seem to have belonged originally to Joy, so while he was certainly a voracious reader I don't have a good idea for how much American lit. he knew, aside from the science-fiction magazines he enjoyed.