I must admit to being nervous about the Avalanche material overall.
No, not just because of the horrible cheesecake art on the covers.
I have their Celtic supplement. They seem to have done an extraordinary job of mixing material from the Gauls, the Welsh, the Irish and Scottish folklore and cherry-picking material from Roman authors (sometimes condemning out of hand, other times accepting wholesale) apparently based on what they wish to present.
But back to Aztecs.
I have heard the GURPS supplement (as usual and, alas, out of print) is pretty good. Of late, however, I have just been searching the 'Net for background material as I want to use the Maya as a basis for an upcoming AU campaign (Giants + Maya Ceremonial Cities = Much Goodness!).
This is a sadly overlooked area for rpg settings -- Aztec, Maya, Toltec, and then stretching down to the Inca, lots of possibilities!
(Slightly OT -- how does one present human sacrifice in a game without making the whole culture inherently evil? thoughts? opinions? corn-fed chihuahuas?)