Overall, I enjoyed it, although it can be harrowing in places. I would class it as a fantasy (or fantastical?) hidden history novel. The framing device of communications between the fictional author and the fictional editor might get a bit odd and wearing but it's worth sticking with as it ties up the fantasy and hidden history nicely.

Source: http://booklikes.com/ash-a-secret-history-mary-gentle/book,1823684