Old Man's War
by John Scalzi

A seventy-five year old widower, John Perry finds the Colonial Defense Force's offer of a new, youthful life intriguing. The CDF fulfills their end of the bargain, now he needs to fulfill his: ten years' commitment as a space marine, fighting the worst the universe has to offer. Funny, thoughtful, and packed with action, Old Man's War is a great read and one of Scalzi's seminal works.