Or son Scott Card, Ender's Game

Now that was a book!

Yes, but Heinlein is my long time favorite but there are a coup,e of modern writers who are excellent. For "hard science" science fiction Ian Douglas is excellent. Start with "Semper Mars". For more of an Horatio Hornblower series there's the Lost Fleet series by Jack Campbell. Both are former Navy. Interestingly the more cerebral writer using more actual physics and theoretical physics is the former enlisted Navy guy who was a corpsman for the Marines... Ian Douglas. jack Campbell was a Naval Academy graduate.
Note: both names are pseudonyms to avoid confusion and stereotyping because they also write in other genres as well.

Thanks for the tips!

I enjoyed Heinlein's The Moon is a Harsh Mistress.

Picked up Courageous by Jack Campbell (The Lost Fleet series) for 2 bucks in a second hand bookshop today. :)