This book is a pretty crazy beast. It would be a perfectly enjoyable science fiction/action novel, if it weren't based around Flavius Belisarius, one of the most fascinating generals of the sixth century and reconqueror of Italy. And it looks like he's going to be up fighting against the Guptas, of all people.
Right before sitting down to write, I've just read through a bit by a grumbling, unhappy semi-sentient jewel from the future (don't ask) which is very upset that Belisarius' famous victory at the Battle of Dara could have been so much simpler if only somebody had just bothered to invent the stirrup. I suppose it does have a point.
A few chapters later, the following charming piece of dialogue is said by Sittas:
"He [meaning Belisarius] probably swore an oath. He's always swearing oaths. Swore his first oath when he was four, to a piglet. Swore he'd never let anyone eat the creature. Kept his oath, too. They say the pig's still around, roaming the countryside, devouring everything in sight. The Bane of Thrace, the thing's called now. The peasants are crying out for a new Hercules to come and rid them of the monster". A belch. "That's what comes of swearing oaths. Never touch the things, myself." (chap 9, pg 160)
The book does love its historical references: Antonina is a major character (her fictional relationship with Belisarius is very different from the generally accepted historical narrative), we see Procopius being hired (and learn why he in particular was chosen), and of course Justinian and Theodora loom over everything. Where I am in the book (page 160), Narses and John of Cappadocia have just been name-dropped. And the authors make a passing reference to -- of all things -- Sailing to Byzantium. Presumably, just because they could.
So: science-fiction/historical/action/fun! An aside: there's a lovely scene early on in which Procopius is caught off guard by some praise from Belisarius. I loved the idea of seeing a famous writer of history flustered, especially since in our world we see those times through his eyes. It's like seeing Shakespeare caught cheating in grammar school, it's a lovely little touch.
(More spoilorific information on An Oblique Approach is available on TV Tropes)