I would like to learn all about the Aztec mythology, yet all of the books that I've turned up in my searches treat Aztec history and culture as a whole, like Smith's The Aztecs. Another promising text seemed to be Leon-Portilla's In the Language of Kings, but it includes many other Mesoamerican cultures in its scope. Others view the religion through a certain historical lens, as Conrad & Demarest do with expansionism in their book Religion & Empire.
I would like to find a text that has a comprehensive study of the mythology of the Aztecs, but it seems like religion and mythology take a backseat role in the most celebrated books. There appears to be arguments over translations as well, notable with the god Itzlacoliuhqui, and I am in no way qualified to discern the quality of Mesoamerican academia in this respect.
I'm not in the US, so if I have to find a book, I'll have to buy it. No libraries offer works like this or inter-library loans where I am.
Does what I am looking for exist?
The best thing for this is a good primary source. The best primary source on the Aztec religion is the Florentine Codex, an extensive study of all aspects of Aztec society compiled in the 1500s by Bernardino de Sahagun and several native american associates of his at the college of Tlatelolco.
Recently, all 12 books were translated into English. The ones you would find most helpful would be:
Book 1: The Gods which is a full description of the entire pantheon and what was believed about each god, complete with illustrations.
Book 3, the Origin of the Gods, might also be helpful, as well as books 4 and 5 on Soothsayers and Omens.
For the mot part, you will see that Mesoamerican religion is often treated as a comprehensive whole rather than segmented into regions or cultures. That being said, because of their popularity and quantity of sources from which to study them from, Maya and Aztec religion are the ones that have received the most amount of academic research. i would suggest you look more into the writings of Miguel Leon Portilla; he is by far one of the most knowledgeable academics in terms of Aztec thought. In particular his book “La Filosofia Nahuatl Estudiada en sus Fuentes” is a comprehensive look into Aztec understanding of the universe. I do not how good the English translation is (Aztec Thought and Culture: A Study of the Ancient Nahuatl Mind), because it does not seem to be a direct translation. Another good person to look into is Alfredo Lopez Austin, in particular his book “Los Mitos del tlacuache” (The Myths of the Opossum: Pathways of Mesoamerican Mythology). While these last one takes a more comprehensive view for all mesoamerica, he pays a great deal of attention to Aztec Religion. In addition, Lopez Austin has written extensively about Nahuatl gods and mythology. I recommend you look at his bio in mesoweb which has a lot of free content. Lastly, take a look at Karl Taube. He has two book that may be of interest to you “Aztec and Maya Myths” and “An Illustrated Dictionary of The Gods and Symbols of Ancient Mexico and the Maya”. These last one takes an encyclopedic approach and is fill with amazing illustrations. These books, plus the already mentioned Florentine codex, are a good start for understanding Aztec religion.