Forever War, by Joe Haldeman - my favorite book. For Hard SciFi fan.
Forever Peace, same author is also good. Not a sequel to the first.
Starship Troopers by Robert Heinlein is also very good (although don't expect it to be like the movie which was based on it).
Armor, by John Steakly.
All of the above are military SciFi.
In the non-fiction area, I liked Inside Delta Force, Eric Haney; Hunt for bin Laden, Robin Moore; Robert's Ridge, Malcom MacPherson, and a whole bunch of others.