If you’re a bibliophile or lover of knowledge, the largest primary source on the Aztecs is the Florentine codex. It’s often the most referenced as well, because it’s comprised of 12 books in total that covers the entirety of the ancient Aztecs. However, it’s very expensive in English, even buying all together a few years back was about 500 bucks if not more so. It’s better to buy them individually, even though it’s a tad more expensive.