It is the second century AD, not that he would call it that, and Photinus the Greek needs to get out of town on account of an angry husband. So, seeing the opportunity to make a few denarii while lying low, he joins a trading expedition over the frontier into unconquered Germany, and what with one thing and another finds himself mispronounced as Votan, and teaching a trick or two to the Aser...
That makes this sound almost farcical, which in part it is; Neil Gaiman's introduction is dead right in comparing Photinus to Flashman, but (at least in the volumes I've read) even dear deceitful Harry never quite managed to pull off godhead. And bastard though Flashman is, there's sometimes a cold inhumanity to Photinus which would make him shiver. Because if the Victorians weren't too particular about the well-being of subject races, they were still positively philanthropic compared to the Roman Empire.
There's another level to it all, though; Photinus is a healer-priest of Apollo, and while some of what he knows we'd call medical science, and other bits conjuring or confidence trickery, there remains a stubborn undercurrent of something stranger. Yes, his visions of Apollo could easily be madness, and yes, sometimes Photinus makes airy assertions which we know with hindsight to be nonsense, like the uselessness of horses for pulling vehicles - but at other times, there are prophecies and spells which seem to work far too well for chance. Is Photinus' story the only wellspring of what will come to be the Norse myths, or is he running on a track that's already laid in the structure of the world? I'm reminded of the myth-as-pattern ideas in Philip Purser-Hallard's Devices books, and I'd love to know whether he read James.
The sequel, Not for All the Gold in Ireland, is based around Celtic myths which I don't know half so well as the Norse ones. As such I doubtless missed many of the story's internal currents - though certain tendencies, such as things often coming in threes and succeeding on the third attempt, are familiar via those legends' later mutation into folk and fairy tales. Many of the same tricks and techniques are deployed as in Votan, some with diminishing returns - but James' endearing and only mildly parodic grasp of rolling Celtic speech patterns makes up for any lack of freshness in Photinus' confidently wrongheaded predictions. There are gestures towards Photinus again taking a divine identity, but they don't seem so fully realised this time; conversely, he witnesses more events which seem incontrovertibly supernatural, especially where the enchanting Rhiannon is concerned. On the whole, it lacks the first book's eerie power, though the scenes set around the battle of Tara, from the haunted night before to the muddy carnage afterwards, have a visceral chill. And having done a little Googling of the tale's background, I know a lot more about Hiberno-Roman relations than I did a week ago (though it seems to be a field where even the experts don't know a great deal for certain).
You could maybe consider Men Went to Cattraeth a sequel too, but only in the loosest sense. It takes place centuries later, in the depths of the Dark Ages, and there's none of Photinus' urbanity here; precious little of his humour, either, though we still get the occasional wry smile at the misapprehensions of an earlier age (dragons are listed with real heraldic animals, unlike the purely mythical elephant). The legions have long departed; superstition and forgotten knowledge are the order of the day. But on the hills and in the woods between the ruined cities, with their half-remembered religion, the Britons still consider themselves Romans, heirs to the likes of the magician Vergil (who raised Hadrian's Wall to impress the emperor's leman Cleopatra). A post-apocalyptic tale, in short, just not one set in the future. And it follows a fairly typical post-apocalyptic trajectory, as a brutalised former bard recounts the assembly of forces from all the British kingdoms into the largest warband any Briton has seen - a full 350 cavalry! - to stand against the invading Savages, who drain the marshes and fell the fruitful forests to plant their barren wheatfields...
There's little enough known about the Battle of Cattreath, but even without knowing that little, you can guess soon enough that nothing here is going to go to plan. Although certain hints and cameos around the margins of the tale do remind us that we are in the vicinity of the Matter of Britain, and a saviour of sorts is coming a few years hence, for all he's worth when you consider that this is is very much Geoffrey of Monmouth's Arthur rather than any of his nobler successors. It's a bleak and bloody novel, all the more so when it takes most trouble to humanise the Saxons. There's one early chapter in particular which recalls the current refugee crisis, as a Saxon dying on the shore explains the dire conditions back in their land of origin which have led to the desperate and expensive gamble of crossing the sea...but one can hardly interpret it as a suggestion that Aneirin and his people should welcome the incomers, given the atrocities on both sides. This is a world where the only thing you can do is stick close by your people and screw the other guy before he screws you; a convincing assessment of its era, but one from which we should probably do our utmost to avoid deriving any contemporary moral.