Wow! And here I thought I was about the only person in the world to have ever read Aztec. Its actually a small series now, three or four of them. The first is still certainly my favorite, being the most detailed in mesoamerica and its many, many varied peoples, practices, and religions. Goes into a lot of detail about their military caste system, with their Eagle Knights and Jaguar Knights and the Flower Wars they fight with nearby rival nations. Good material for an aspiring modder for Mount&Blade, god, I'd love to see that.
The subsequent books in the series are also very good. I'm not a huge history buff, but I believe most of it is accurate with what we actually know of history (remember; whoever wins the war writes the history books and the brave Mayans and Aztecs certainly did not come out on top). The second book (Aztec Autumn) follows the chronicles of the son of the main character from the first book. The third book (Aztec Rage) follows the story of a Gachupine, a Spanish "wearer of spurs" and rivals the first book in its awesomeness for pure swash-buckling badassery.
The whole series is certainly a must read for any fan of ancient war and battle, and certainly of interest to anyone who enjoys learning about the early americas but shies away from drab and dull history books.