Double Star is a science fiction novel by Robert A. Heinlein published in 1956. It's a Ruritanian Romance - specifically, a down-on-his-luck actor is asked to temporarily double for an unspecified public figure for a couple days. Ruritanian Romances usually centre on the idea that some member of royalty has a visual double out there in the world, and the affairs of the two become entangled.
In this case, our first-person voice is "The Great Lorenzo" (actually "Lawrence Smith"), the actor in question. He's direly in need of money, so he accepts the job - only to find out that it's extremely dangerous and involves Martians, who he hates.
The politics are Heinlein - free trade, expansionist, and mildly sexist (although he thought he was being open-minded). Lorenzo's voice is initially so spectacularly conceited and self-centred that I really hope this was intended as a teen novel: it's unnecessarily over-the-top. But it develops nicely, and the story is pretty good: there's a reason his name is still remembered to this day. He was a good writer.