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place in post-war England, and they specifically bring in references to Tolkien's legendarium (at that time largely unpublished) and treats these references as primary fact within Lewis's fiction.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:
On Fairy-Stories
Tolkien was among the pioneers of the genre that we would now call fantasy writing. In particular, his stories—together with those of C. S. Lewis—were among the first to establish the convention of an alternative world or universe as the setting for speculative fiction. Most earlier works with styles similar to Tolkien's, such as the science fiction of H. G. Wells or the Gothic romances of Mary Shelley, were set in a world that is recognisably that of the author and introduced only a single fantastic element—or at most a fantastic milieu within the author's world, as with Lovecraft or Howard. Tolkien departed from this; his work was nominally part of the history of our own world, but did not have the close linkage to history or contemporary times that his precursors had.