@maxdancona,
Based on your other titles:
Old Man's War by John Scalzi.
Quote:Old Man's War is a military science fiction novel by American writer John Scalzi, published in 2005.[1] His debut novel, it was nominated for the Hugo Award for Best Novel in 2006.
Based on reading Heinlein,
Starship Troopers. Note, the book is mostly about social responsibility and why men go into battle. They had to strip all that out for the movies, so you would get a lot from the book even if you'd seen the movie.