I think there's a pretty decent chance that Lewis read Lovecraft's opus At the Mountains of Madness as serialized in Astounding. It seems virtually certain that he did read the magazine, since Wandrei's "Colossus" pretty much has to be the story Lewis referred to in his preface to his book The Great Divorce (see above), and that story would have been available only in the magazine.
I doubt very much that either man ever read an issue of Weird Tales. I'm not sure if it was available in England. Neither Tolkien nor Lewis seems to have liked horror for its own sake. The sleazy covers of WT would have put both off, supposing some store even displayed them.
Lewis married an American woman. She died before he did. After his death, his library was found to include the Arkham House book The Opener of the Way by Robert Bloch. I imagine it was one of Joy's books. Whether Lewis ever sampled it or what he thought of it if he did, I don't know. I doubt he would have cared for it. But At the Mountains of Madness (or "The Shadow Out of Time," also in Astounding, even more than Mountains) would, I suspect, have impressed Lewis, a great fan of the fiction of Stapledon, Wells, &c.
Ray, as regards the availability of American sf pulps, here's some material from another of my articles:
Is Lewis’s Ransom Trilogy Indebted to
“Yank Magazine” Science Fiction?
C. S. Lewis’s “On Science Fiction” was read, or was the basis of a talk, at a 1955 session of the Cambridge University English Club (Hooper xix). In this paper, Lewis said that, “some fifteen or twenty years ago,” he “became aware of a bulge in the production” of stories of the type pioneered by H. G. Wells. “In America whole magazines began to be exclusively devoted to them” (“On Science Fiction” 55). This statement nails down the fact of Lewis’s reading of American pulp “scientifiction.” More specifically, historian of science fiction Mike Ashley regards the “mid-thirties” as the time in which these magazines exhibited a phase of “cosmic sf,” emphasizing stories that dealt with “not just the exploration of space but the nature of time, space and the universe” (231). Ashley’s “cosmic” science fiction correlates with at least one of the “subspecies” of science fiction that Lewis said appeal to him, stories of the “Eschatological,” which evoke “our collective smallness” and the “slow biological, geological, and astronomical processes” that may, “in the long run,” make many of our hopes and fears “ridiculous” (“On Science Fiction” 61-2).
Both chronologically and thematically, the two stories discussed in the present article, and also Wandrei’s “Colossus” (see below), conform to Ashley’s and Lewis’s descriptions. The fact that the magazines in which they appeared were American, while Lewis lived in England, is no reason to doubt that Lewis could have read and been significantly influenced by these three stories. The ready availability to British readers of such magazines is attested by Richard Kyle, who recalls “bins of ‘Yank Magazines – Interesting Reading’ in the English Woolworth stores of the middle ‘30s” (Lupoff 92).
[rest of article omitted; partial bibliography follows]
Ashley, Mike. The Time Machines: The Story of the Science-Fiction Pulp Magazines
From the Beginning to 1950. Liverpool: Liverpool University Press, 2000
Lewis, C. S. “On Science Fiction.” On Stories and Other Essays on Literature. San
Diego: Harcourt Brace, 1982.
Lupoff, Richard A. and Patricia E. Lupoff. The Best of Xero. San Francisco: Tachyon,
2004. This book is a selection of contributions to a classic fanzine of 1960-1962
I don't know, Ray. It just seems highly probable that Lewis became aware of one or more of the American sf pulps by the mid-1930s and, having begun to read some issues, kept on doing so for the rest of his life. (He contributed to several issues of The Magazine of Fantasy and Science Fiction, by the way.)
I've wondered if he sometimes passed on issues to Tolkien or others. I don't think he kept the issues. Thus, as I recall, in the Aldiss-Amis conversation he mentions a story that impressed him but whose author and title he can't remember. It's pretty certainly one of Zenna Henderson's stories from F&SF. In an article on sf he mentions a little-known story from F &SF by author and title, so perhaps he saved that issue, or tearsheets anyway -- "Cast the First Shadow."
He was really well-read in sf and fantasy, although I suppose a lot of that reading came after he had written his sf novels. His example, of an Oxford and then Cambridge don who was public about his enjoyment of such literature, must have been encouraging for British writers who wrote relatively early books about sf (Amis's New Maps of Hell, Aldiss's Billion-Year Spree, etc.).
I'm ranging away a bit from Aldiss, but by the way it was interesting to compare the list of books in the Ballantine Fantasy series that Lin Carter edited 1969-1974 with the catalogue of Lewis's library prepared after his death, and to see how much overlap there was!
Here's that catalogue -- keep in mind it was made several years after his death and doesn't list everything he'd ever owned! But look at some of those holdings for classic sf and fantasy.
By the way -- avoid at all costs the book of Lewis-Clarke letters (called something like From Narnia to a Space Odyssey). It is perhaps the worst-edited book I have ever seen, in that again and again the Lewis letters are transcribed so as to make no sense. If you want to read Lewis's letters to Clarke, they are available in the appropriate volume(s) of the Collected Letters (a 3-volume set). In those books too you will find plums such as Lewis's fan letters to Mervyn Peake, his arch correspondence with E. R.Eddison, etc.