Since I read Redshirts earlier this year, John Scalzi has been becoming one of my favorite sci-fi authors. Old Man's War is an extremely well crafted story. It borrows heavily from Robert Heinlein's Starship Troopers for the aesthetic. You've got a well-trained army going against a thousand things that would eat them for breakfast without hesitation. Then you get into a bunch of nature versus nurture questions about behavior and what it means to be human. Old Man's War is an exquisite piece of contemplative fiction.