This Immortal
by Roger Zelazny · 1966
Buy on AmazonFollowing a nuclear war, Earth is sparsely populated, and owned by a race of blue-skinned aliens called Vegans. Conrad Nomikos has been assigned to show an influential Vegan around the old ruins of Earth. When attempts are made on the alien visitor's life, Conrad realizes he needs to keep Cort Myshtigo alive, and find out why.
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Hugo Award for Best Novel — Winners · en.wikipedia.org