If you're new to science fiction, then there is of course a huge back catalogue of SF to catch up with, much of which is available in ebook formats.
A couple of books which might fit your requirements:
The Mote in God's Eye -- Larry Niven & Jerry Pournelle -- Human space empire in first alien contact. There's a sequel called The Gripping Hand, but I can't recommend it because I haven't read it yet.
Downbelow Station -- C.J. Cherryh -- Peaceful orbital station becomes refugee camp in the middle of an interstellar war. There are lots of other books set in the same fictional universe, of which my favourites are Merchanter's Luck and Forty Thousand in Gehenna. But it helps to read Downbelow Station first.
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