Old Man's War
Fans of Robert Heinlein's Starship Troopers and Joe Haldeman's The Forever War will enjoy John Scalzi's 2005 novel Old Man's War. The first in a continuing series (with the most recent installment, Zoe's Tale, available here), Old Man's War focuses on the exploits of John Perry, a 75 year-old-man who, sometime in the far future, enlists in the Colonial Defense Forces, where, outfitted with a new, younger and enhanced body, battles various hostile aliens. Perry also discovers that his late wife...but that would be telling. Author Scalzi's novel is an exciting action/adventure thriller with lots of intriguing tecnology (such as "skip drives" and a very cool space elevator), yet doesn't stint on characterzation or dramatic conflict. Recommended.