Old Man's War by John Scalzi
This is one of the few books/series with war as a prominent theme that I truly enjoy. Scalzi writes witty amusing characters, who are an absolute joy to read.
John Perry is old, and he is being recruited for war in outer space. In this first book, you get to learn about this new world where it's better to have old people fighting than young people. It's not your typical space opera especially with Scalzi at the helm.
The action is interspersed nicely with character development, and the depth, richness, and mild sarcasm of the author's imagination makes this book a great read.
If you like sci-fi, you should read this. If you don't like sci-fi, you still might find yourself enjoying this book.