Codices are traditional Mesoamerican books. The Aztecs would paint beautiful constellations of pictorial glyphs on sheets of amatl paper. Instead of rolling up the paper like a European-type scroll, they would be folded accordion-style. These books were created by priests and scribes who were skilled in the arts, and they preserved a great body of learning in their pages. Unfortunately, only a handful of books survived the destruction of the Conquest, and their translation is still a work in progress. FAMSI has commentary (none in English, sadly) on many of these books, which you can reach from the main index for Aztec codices at FAMSI below. Codices hosted by other sites may have commentary, which will be noted below as appropriate.
Index Of Codices