This is probably the best book you can buy to get a good academic introduction to the Aztec Triple Alliance and their conquests. The book itself is written well (you won't fall asleep reading it), and the information contained within is excellently sourced and supplemented by pictures, maps, and a glossary of Nahuatl words. Hassig goes into great detail about the Aztec "home front" and does a great job explaining the nuances of mesoamerican politics and political control (one would hope so, considering the title). After that, you get a king by king and later emperor by emperor account of Aztec military history and a thorough, fair explanation of why and how the Aztecs were defeated by the Spaniards. Would recommend to anyone as a jumping off point for learning about the situation in late postclassic mesoamerica, as there really isn't a better book (that I know of) for this subject. ( on line review)
Ross Hassig, a historical anthropologist specializing in Mesoamerica, is the author of Time, History, and Belief in Aztec and Colonial Mexico; Aztec Warfare: Imperial Expansion and Political Control; and Trade, Tribute, and Transportation: The Sixteenth-Century Political Economy of the Valley of Mexico.