Inca, by Blom
I was at the Worldcon, the World Science Fiction Convention, over the Labor Day weekend, in Boston, and picked up the book from the author.
It's good. Made me think I really was in their time. It's told from the point of view of one of the Inca's sons, a potential successor. He made a bid for the Inca in OTL and won, but in this ATL he is sidelined and spends several years learning about the Spanish instead of conducting the civil war against one of his brothers. This sets him up for the sequel, which I will buy.
I doubt it's authentic because while I'm sure the author did her research well, there really isn't much research to do because the source documentation is so sparse. The Spanish weren't much on writing things down. Especially things that didn't have to do with money.