The ending is intentionally vague.
To me it's clear there is no right or wrong answer to whether Kevin Spacey was (or was temporarily inhabited by) an alien or not. The writers have intentionally supplied plenty of augments for and against, and the whole point in this story (disregarding any other books that came after) is to make you think about it and discuss it, and to think about the other messages in there too. For example, the importance of making the most of family and avoiding regrets, which is why the doctor patched things up with his estranged son at the end.
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