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.
Mount Doom
Reviewer Bart Higgins noted the similarity between Tolkien's Mount Doom and the volcanic caverns under Venus depicted in "Perelandra" by Tolkien's friend C.S. Lewis. "In both cases, the book reaches a cataclysmic scene of a struggle in an underground cavern, ending with an evil being falling into volcanic fire and being consumed. Since it is well known that Tolkien and Lewis frequently shared with each other their respective works in progress, it is reasonable to assume that the resemblance is no coincidence".