A total of 12 books were written. Each on a different topic. The original purpose was to write the books in order to understand those who did the ways and to easily convert natives.
To give you some context:
Book 1 is about the gods:
Though the Nahuatl text has more detail, the Spanish version tries to compare the Nahua gods with Greek and Roman mythology to make it easy to European readers.
Book 2 is about Ceremonies:
Book 3 is about the origins of the gods:
Book 4 is about a person's destiny based on their day sign:
Book 5 is about omen ans superstitions:
Book 6 is about rhetoric and moral philosophy:
This includes the famous talk of advice told by a Nahua father to his daughter.
Book 7 is about the Sun, Moon and Stars:
It has the famous 5th sun creation story here.
Book 8 is about the upper class:
This includes rulers and nobles, forms of government, elections, the customs and pastimes of the nobility.
It later goes on about how clothing was made and textile patterns worn by nobility. The majority of Ancient Nahuas could only wear clothes made from agave threads not dyed and without adornment, but the nobles wore colored cotton clothes decorated with shell or bone-and-feather patches.
Finally a bit about the ball game.
Book 9 is about merchants:
This includes information about merchants, officials responsible for precious items and feather working.
Also, it includes information about smoking which was done during gatherings and religious ceremonies, using pipes that were filled with herbs and certain grasses or by smoking cigars made by rolling up tobacco leaves.
Book 10 is about society:
This includes society and covers such as the virtues and vices, food and drink, the parts of the human body and sicknesses and cures.
This includes the process of making the chocolate drink from cacao beans, agriculture and food preparation.
Also, this has a famous story: Quetzalcoatl & the Toltecs. There's an interesting summary of the cultural evolution of the Nahuas.
Book 11 is about natural history:
The largest of the book. Pretty much a pharmacology book. Animals based on Mexica belief.
Book 12 is about the conquest of the valley of Anahuac:
Eventually I'll go more into details about each book in the future.