This is a book that has stuck in my mind for over a decade since first reading it. It's one of the essential, classic works on Mesoamerican civilization, and despite the passage of many years since its publication, it is still one of the most readable accounts of Aztec life. The focus of the book is on life in Tenochtitlan, supplemented with material on other Nahuatl-speaking cities (Texcoco, etc), and not on the archaeology or wider history of Mesoamerica. It is an intimate portrait rather than an overview, and as such it has lost little value over time. The details of the development of Aztec civilization as presented by Soustelle are broadly correct, and the understanding of everyday life can hardly have progressed over the decades, especially as most of our sources have remained the same (especially Bernardino de Sahagun). This works as an excellent primer on the Aztecs, and it would be a good choice for a world history or anthropology reading list. Reading this before embarking on Inga Clendinnen's or Caroline Dodds Pennock's more recent works will allow those latter to fill in the blanks and to flesh out the picture provided by Soustelle's easily-readable, erudite, fascinating book.
Soustelle was not simply interested in the Aztecs, and wrote several articles on non-Nahua topics, including Otomi life and society. This means that his account allows for the diversity of people who must have lived in and visited Tenochtitlan and the Valley of Mexico in pre-Cortesian times - and this really brings that world to life.
This is simply one of the best introductory works to any human civilization, and I can't recommend it highly enough.