For starters, here's an album of some images I grabbed online as reference for the style of my atlas (the style the explorer would use, not the native style): http://flickr.com/photos/dihughes/sets/72157602256257858/. Next week I will have scanned a lot of stuff from the books I'm reading to use as reference for the art style of the inhabitants of Altepehua.
Thanks to the generosity of our dear Marientina, I have a large stack of books that I'm wading through as I'm researching native North and South American cultures. A welcome supplement to my largely Wikipedia-based research which, though a good place to start, really should not be the only place I look. That said, here are the books I'm working my way through.
Pre-Columbian Art, Esther Pasztory
Native North American Art, Janet C. Belo and Ruth B. Phillips
Peru Before Pizarro, George Banks
Moon, Sun, and Witches: Gender Ideologies and Class in Inca and colonial Peru, Irene Silverblatt
Native American Art in the 20th Century, W. Jackson Rushing III
Women in Ancient America, Karen also Bruhns and Karen E. Stothert
The Frieze of the Palace of the Stuccoes, Virginia E. Miller
Arts and Crafts of South America, Lucy Davies and Mo Fine
The Art of Mesoamerica: From Olmec to Aztec, Mary Eller Miller
Moche Fineline Painting: Its Evolution and Its Artists, Christophe B. Donnan and Donna McClelland
The Conquest of the Incas, John Hemming
Many many course readers and papers, Marientina Gotsis