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place in post-war England, and they specifically bring in references to Tolkien's legendarium (at that time largely unpublished) and treats these references as primary fact within Lewis's fiction.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:
Roy Campbell (poet)
Tolkien, who was then hard at work writing "The Lord of the Rings", was charmed by Campbell. In a letter to his son Christopher, Tolkien compared Campbell to Trotter, a torture-crippled hobbit character who appeared in an early draft of "The Lord of the Rings", to become the character of Aragorn in the final versions. Tolkien further commented, Here is a scion of an Ulster prot. family resident in S. Africa, most of whom fought in both wars, who became a Catholic after sheltering the Carmelite fathers in Barcelona — in vain, they were caught & butchered, and R.C. nearly lost his life. But he got the Carmelite archives from the burning library and took them through the Red country. [...] However it is not possible to convey an impression of such a rare character, both a soldier and a poet, and a Christian convert. How unlike the Left – the 'corduroy panzers' who fled to America [...]