Like you, I am interested in the Battle of Teutoburg Forest. Here is two recommendations: The Battle that Stopped Rome by Peter Wells and Give Me Back My Legions: A Novel of Ancient Rome by Harry Turtledove. Wells offers a good overview, while Turtledove offers an an interesting fictional look at the relationship between Varus and Arminius that was so pivotal in Rome’s greatest defeat. Also Turtledove’s book is NOT part of a historical fiction series so you are not required to buy a ton of books. Hope you enjoy the books!!