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place in post-war England, and they specifically bring in references to Tolkien's legendarium (at that time largely unpublished) and treats these references as primary fact within Lewis's fiction.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:
Christopher Tolkien
Tolkien had long been part of the critical audience for his father's fiction, first as a child listening to tales of Bilbo Baggins (which were published as "The Hobbit"), and then as a teenager and young adult offering much feedback on "The Lord of the Rings" during its 15-year gestation. He had the task of interpreting his father's sometimes self-contradictory maps of Middle-earth in order to produce the versions used in the books, and he re-drew the main map in the late 1970s to clarify the lettering and correct some errors and omissions. J. R. R. Tolkien invited Christopher to join the Inklings when he was twenty-one years old, making him the youngest member of the informal literary discussion society that included C. S. Lewis, Owen Barfield, Charles Williams, Warren Lewis, Lord David Cecil, and Nevill Coghill.