Give Me Back My Legions!, by Harry Turtledove. This is historical fiction, without any elements of alternate history or science fiction. I liked it much better than most of the Amazon reviewers did. The story kept moving, and built up the sense of impending doom.
Little details stand out. Stirrups had not been invented, so getting on a horse, and staying on one in battle, was a lot harder. Correspondence addressed to older people was written with large letters so they could read it. There were no glasses.
Slavery was practiced by both cultures, but there were different attitudes. A Roman master was more likely to beat his slave, but a German was more likely to kill his. German society was freer: Taxes were a Roman innovation for them. However, Germany was a more violent space. Quarrels were settled by the sword rather than by lawsuits.
Having finished the novel, I am now reading The Battle That Stopped Rome: Emperor Augustus, Arminius, and the Slaughter of the Legions in the Teutoburg Forest. This has happened many times in my life: I read a novel with some particular historical setting, then I read non-fiction to learn more about the period. I have learned a lot history this way. It started about 45 years ago with Lest Darkness Fall. I was a teenager when I read it, and I must confess that part of my motivation then was to learn what the Empress Theodora complained about :-)> See the Secret History by Procopius. NSFW.