Edgar Rice Burroughs wrote books in a number of different settings:
- Tarzan (jungles of Africa)
- John Carter of Mars
- Carson of Venus
- At the Earth's Core -- Pellucidar (inside the Earth)
- The Moon Series
- The Caspak Series
- The Mucker Series
Along with some westerns, historical novels, and more.
I know that Tarzan, John Carter, and the Pellucidar series are all in the same universe, since Tarzan has visited Pellucidar, and Jason Gridley, from the Pellucidar books, is able to contact Barsoom with Gridley waves.
Are all of his different worlds and series in the same overall universe, or were some of them in a separate universe?