J. R. R. Tolkien
The request for a sequel prompted Tolkien to begin what would become his most famous work: the epic novel "The Lord of the Rings" (originally published in three volumes 1954–1955). Tolkien spent more than ten years writing the primary narrative and appendices for "The Lord of the Rings", during which time he received the constant support of the Inklings, in particular his closest friend C. S. Lewis, the author of "The Chronicles of Narnia". Both "The Hobbit" and "The Lord of the Rings" are set against the background of "The Silmarillion", but in a time long after it.
Boxen (C. S. Lewis)
Boxen is a fictional world that C. S. Lewis ("Jack") and his brother W. H. Lewis ("Warren") created as children. The world of Boxen was created when Jack's stories about Animal-Land and Warnie's stories about India were brought together. In "Surprised by Joy", Jack explains that the union of Animal-Land and India took place "sometime in the late eighteenth century (their eighteenth century, not ours)".