H. Beam Piper Paratime/Lord Kalvan of otherwhen
RH Heinlein Tunnel in the sky, Glory Road, anything you can find, including the "Heinlein Juveniles".

Oddly enough, western novels by Donald Hamilton, Loren Estleman, or Louis L'Amour hit the same notes for me: solid science, natural history, cultural history, interesting weapons and compelling landscapes.
Sidenote: none of these authors had a character pick up a revolver that turns into a semi-auto later in the paragraph. Some authors do that. Makes me crazy.
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Dance like you have never been hurt, work like no one is watching,love like you don't need the money.