I'd be interested in knowing about them from a non biased source.

Sorry to disappoint you, but that's not possible. For one, there's no such thing as a non-biased source, but specifically in this case there are exactly zero definitively pre-Columbian Aztec codices. There are surviving Aztec codices, but they date to the early colonial period after Spanish arrival. There are pre-Columbian codices, like the Borgia group, but they're not really "Aztec" in the sense that they largely come from the Eastern Nahua and are largely esoteric religious texts anyways. Thanks to a campaign of Spanish book-burning, all of the pre-Columbian Aztec books were destroyed.

The closest you can get to a "non-biased" source on the Aztecs would be archaeological evidence, but even that is biased in the sense that some things preserve over time and others don't. Plus, although we don't like to admit it, archaeologists have their own biases that may color how they interpret the data.

That said, there is a tremendous amount of information on the Aztecs from colonial era sources like Duran and Sahagun. After having destroyed all the books, the Catholic clergy operating in New Spain realized that if they want to convert the natives they really need to understand native culture. Chroniclers like Duran and Sahagun went out of their way to detail Aztec history, culture, religious beliefs, etc. They worked with natives to compile this information. Now granted, they are biased, but all historical sources are biased. This is why historians don't just take primary sources at their word, and compare what different sources are saying and try to situate what the source is saying within the context of that bias.

So ultimately, if you want to learn reliable information about the Aztecs, I would say you're better off going with a book written by a modern historian or archaeologist than reading a primary source. Modern researchers have already done the hard work of sorting through the primary sources to separate truth from exaggeration.

For recommendations, I'd point you to the AskHistorians book list which has an extensive list of good books on pre-Columbian civilizations in Latin America.