Middle-earth
Another example is C. S. Lewis's "Space Trilogy," in which Earth is also called Middle-earth. Lewis's novels are situated at the approximate time of World War II, with the last novel, "That Hideous Strength," taking place in post-war England, and they specifically bring in references to Tolkien's legendarium (at that time largely unpublished) and treats these references as primary fact within Lewis's fiction. Merlin, of King Arthur fame, is treated as a successor to the Atlantis magic found within "Numinor" (Lewis's unintentional misspelling of Númenor), and similarities can also be found in the Quenya name for Númenor, which is "Atalantë," and Lewis specifically references the earth as Middle-earth twice, both in Chapter 13, "They Have Pulled Down Deep Heaven on Their Heads". Lewis's borrowings came directly from Tolkien himself: they were close friends and shared each other's literary creations. Lewis encouraged Tolkien to persevere with his Middle-earth writings.A small selection from the many books about Tolkien and his created world:
Edward Tangye Lean
After leaving Leighton Park School, Tangye Lean was an undergraduate at University College, Oxford. While at Oxford, he founded the original Inklings club around 1931, for the reading of unfinished compositions. The club consisted of students and dons, including J. R. R. Tolkien and C. S. Lewis. When Lean left Oxford in 1933, the club faltered. However, its name was transferred by Tolkien and Lewis to their literary group at Magdalen College. Lean also contributed to the "Isis" magazine at Oxford.