In my mind, there are two kinds of Gurps books. Those with new rules and new systems to handle new situations, and those that just provide background information for a certain genre. Aztecs is a book of the second kind.
I have to say that I am biased heavily towards the first kind of book. If you study up on Aztecs, you could run an Aztec campaign just fine without owning this book, and if you don't know a fair amount about Aztecs then it doesn't seem to likely that you are going to be trying to build a campaign around them.
Like most GURPS books, it is well written and well researched. If you like to read GURPS books for inspiration, and to learn things, you will probably like this book.