Steve Stirling’s latest: time travel back to Roman Empire, just before things went really, really bad for it. Read it quickly once already, am now savoring it a second time. Note: it is good, fast-paced, and utterly unsentimental over how bad the Roman Empire can be, once you got on its bad side. Mind you, the German tribes in the Marcomannic Wars were a good deal worse.
You can grab TO TURN THE TIDE over at Baen, or wait for it to come out on Amazon. Dealer’s choice.