All You Need is Kill by Hiroshi Sakurazaka
The easiest way to explain the plot is Groundhog Day (the Bill Murray movie) mixed with Starship Troopers (the book). The protagonist of the novel is Keiji Kiriya a rookie drafted into the defense force to fight against the invading mimics a mysterious alien force. Unfortunately for our hero he is killed on his first mission out of basic training but for some unknown reason he wakes up the morning before he is killed back in his bunk. He eventually becomes an excellent soldier and encounters a young woman named Rita known through out the world as the Full Metal Bitch, the most decorated soldier of the war.
It is a very fun read with likable characters and kick ass battle scenes. The story it's self is intriguing to say the least and the reasons behind the time loop are actually explained somewhat and seem to make sense. For those of you less inclined to read a movie is currently planned with a 2012 release date and if done right could be a great blockbuster popcorn flick! You can find the book at amazon and other online book sellers and perhaps even your local book stores but don't get your hopes up at finding it at your local library it just does not seem like something they would have picked up on a whim.