Charlie shows that he can write heroic fantasy as well as everything else. Except, of course, it isn’t really fantasy. When your hero discovers she can switch at will (or “world walk”) between the “real” world (present-day America) and an alternative world (geographically similar, inhabited, but never had industrialisation) then what you are dealing with seems a lot more like SF to me.
Of course, the alternative world works on a feudal system, and weapons are mediaeval (apart from ones that have been carried over from “our” world). So it has some of the tropes of fantasy, and more may develop. But it looks like there won’t be any magic other than the world-walking ability.
The main fault with it is that it shows its history as the first part of a much longer book which the publishers decided should be split in two. So just as it’s starting to get really interesting, it ends.
Oh well, I look forward to reading the second part, and its sequels.