I've been meaning to mention this book for a while — I read it a few months ago and found it to be a lot of fun. It's very different from Old Man's War; both are well-written, but Old Man's War (which was written after Agent) is more serious and generally meatier. Still, Agent is a pleasant read, and I much enjoyed it. If you want to choose one of the two, choose Old Man's War; but both are worth reading.

UPDATE: InstaPundit, who recommends this book, too, reminds me that you can check out the text free here.