The Battle that Stopped Rome - Emperor Augustus, Arminius, and the Slaughter of the Legions in the Teutoberg Forest by Peter S. Wells (nonfiction 2003)

I've read and enjoyed Wells' The Barbarians Speak, and this one is very exciting too.  While there isn't much new or different in the part about Augustus, the sections on Varus and Arminius are wonderful.  The reconstruction of what happened at the battle based on the extensive evidence is absolutely gripping, horrifying, amazing.