I'm reading The Fellowship: The Literary Lives of the Inklings ... It's very interesting to read about the friends of
Tolkien and how they influenced one another.
For the Inklings, I very much recommend the book by JRRT's official biographer, Humphrey Carpenter, "The Inklings" from 1978. It concentrates on the three "major Inklings", C.S. Lewis (the "gravitational center"), JRRT and Charles Williams. Carpenter mentions that there have been several writings on this supposed influencing each other, but concludes that any actual influence had been rather in a single direction. While JRRT and Lewis were Oxford dons, Williams worked for the Oxford University Press in their London offices. He and his colleagues had moved to Oxford in September 1939 due to WW II. Williams and Lewis had first made contact a bit over three years before. By the time Williams moved to London in 1939, he was 53 years old, while JRRT was 47 and C.S. Lewis not quite 41. The two older
men, for several reasons besides their greater age, were not the types to be influenced easily - least of all by Lewis. Lewis at least once commented explicitly on this being difficult with regard to JRRT. The other way around, JRRT once commented that Lewis "was a very impressionable man". One indication of this, and Lewis shifting his "allegiance", can be seen in the discrepancy between the first two
books of his science fiction "Cosmic (or "Space") Trilogy", "Out of the Silent Planet" from 1938 and "Perelandra" (also titled "Voyage to Venus") from 1943. While the third volume "That Hideous Strength" from 1945 is much closer in publication date to "Perelandra" than that to the first book, the third one differs from the first two in almost every conceivable way. Not in the least that THS, at just over 400 pages (in my single-volume paperback) is longer than OOTSP (144 pages) and P/VTV (200 pages) combined. Don't let that middle book's publication date fool you. Lewis wrote fast, and, to shift to his better-known publication, "The Chronicles of Narnia", he had finished that long before sensible publishers actually brought those seven books to the market. And during WW II, publication of books (every conceivable thing was rationed in the UK during that time, not the least paper) was on a way-back burner.