You can read the full Locus Review on the Mayan December main book page, but here are the highlights:

Switching between perspectives that include Alice’s educated skepticism, Nixie’s attention to details, and some early Mayans’ very different notions of their world, Mayan December achieves the unlikely feat of making a novel about time travelers at a crucial (shared) moment not just vivid but convincing. It never loses all connection with sanity or hope, despite threats ranging from the possibility of wholesale collapse for two great civilizations to more specific elements like Mayan religious rites of human sacrifice…Cooper handles one further complication, an international conference that brings world leaders to the Yucatan, through a deft interplay of scenes in the two eras. How is an archaeological tour for VIPs like a ritual dance for one’s own fetish beast? Mayan December provides some provocative answers.