This is probably a three and a half star. I waffled about giving it four but in the end I couldn't. There's some great subtext here about the universality of myth and how it intersects with children's fiction, and Jungian archetypes, and victorian hubris. If it had all clicked together this would have vaulted into five star territory. But it never quite clicked.
Part of the problem for me is that it couldn't quite find its tone. Some other reviewers have said that the characters seem unreal because they don't show the emotional responses to events that one would expect of them. Thing is, the book is a blending of myth and realism and in myth and folklore characters don't act the way they do in the grocery store. So if the characters are mythic, then they aren't going to have the reactions we might expect. Isis doesn't have a nervous breakdown and get a prescription for anti anxiety medication when Seth chops up her husband, she goes looking for the parts.
Of course the whole point is that they are both - myths and regular folk. But somehow that doesn't gel. The characters end up being these confused neither fish nor fowl mixtures that don't ring true as regular folk or as mythic figure. Also the plot is really just a device to force the characters to wander around in the Nefer Lands. It really isn't strong enough to carry the book - and sometimes it teeters on the edge of just being a travelogue of this underworld that she's invented.
So even though this had the potential to be really outstanding, the elements didn't come together and it ended up just being a fun day trip to the outskirts of the perilous lands. Which is not so bad, really.