Just some of my favourites:
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The Forever War - Joe Haldeman: Basically the vastly better Starship Troopers. Just don't read to sequel.
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Old Man's War - John Scalzi: Very much feels like an modern update of The Forever War
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Cities in Flight - James Blish: I just love the bonker's idea of whole cities declaring their independence from Earth and just buggering off into space
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Ubik - Philip K. Dick: Hard to pick a favourite Dick, but this one just has all the mindfuckery and weirdness of that particular author in a perfect blend