An sf writer called Bob had had an idea for another Mars book about a human orphan adapted by Martians but that will not now be written. His earlier book had involved oxygen masks and skating on the canals.
What is a "Mars book"?
Red Planet is set on Mars.
War Of The Worlds is set on Earth because it is about Martians invading Earth.
Stranger In A Strange Land is set on Earth because it is about that orphan "returning" to his parents' home planet.
Red Planet and Stranger... have the same Martians.
A book can be about Mars, Martians or both.
