The Aztecs by Michael E. Smith
This was a decent book I bought cheaply used. Smith is a scholar and archaeologist, and gives a good general overview of the Aztecs. They actually have quite a bit in common with Rome: a warrior civilization, a decentralized empire, engineering capabilities, the centrality of religious rites to the preservation of the state. One fascinating chapter does go into detail about the human sacrifice they practiced.
Strange fact: Aztecs weren't big meat eaters, but when they did eat animal flesh, small dogs were on the menu. And you thought only Koreans did such things!
The Incas: People of the Sun. Carmen Bernard
Not as academic as I had liked, more of a travel-sized, coffee table presentation. But easy to read, great photographs, and I bought it very cheap from a used dealer.
Like the Aztecs, the Incas bore some similiarities to the Romans, only more so. An emperor at the head, an efficient bureacracy, and impressive engineering feats (especially in a public roads system and a series of fortifications and temples).
The Incas also had their version of the Vestal Virgins: girls who were picked at a young age to serve religious duties for most of their lives, and if they broke their vows of chastity they were buried alive!
And, oh yes, they practiced human sacrifice.
An interesting book.
Next up: the Mayans.