Reddit reviews A History of Western Astrology Volume I

Astrology has evolved over a long period of time and has hopped to different cultures within that period of time. The first semblance of astrology that we really have is Babylonian in origin, but it's very, very different from how we practice astrology today. Babylonian astrology mostly focused on astral omens. So, the priests would look up and see something in the night sky and then derive meaning from that. The planets were seen as the seats of specific gods, and by tracking their motion through the sky we are able to get an idea of what that god is up to. Babylonians also introduced constellational figures, and many of them are similar to what we have now, some are a little different. You can read more about that here.

From Babylon astrology went to Egypt where it retained much of its meaning. The Egyptians systematized it more and introduced concepts like the Hour Watchers (which would ultimately become the houses), calendrics (with Decans and helical rising) and perpetuated the Babylonian religious and magical practices.

As Alexander the Great was trying to conquer the world, a lot of information and customs from other places were sent to Greece, and this is where we enter the Hellenistic period of astrology. The Greeks really systematized astrology into a way much more recognizable of how some practice astrology today (I say some because the way most people, especially online, think about and practice astrology is so very different from the way it's been done historically) by introducing concepts like the horoscope (the Ascendant degree), houses, aspects, basically everything.

Here is a good set of books about the history of astrology

To answer your question, specifically, about the characteristics of the signs. In historical astrological texts, the signs aren't described that way. They are talked about as a list of a series of characteristics. Aries, for example, is the ram, masculine, hot and dry, fiery, choleric, moveable, straight, semi-barren, bestial, the house of Mars and exaltation of the Sun. This isn't that much unlike contemporary websites saying that Aries is "a masculine, cardinal, fire sign", but the classical way contains a lot more nuance. Each of those words means something about the way the sign operates and can be used in various contexts, but they all come together to paint a picture of what the sign is and does. So, why is Aries aggressive? Primarily because it's the sign of Mars, just like Scorpio. What makes it more overtly aggressive than the aggression of Scorpio is because Aries is moveable, fiery, and bestial, which takes those impulses of violence and pushes them out whereas the coldness, femininity, and fixidity of Scorpio allows it to be held in.

• The Heavenly Writing: Divination, Horoscopy, and Astronomy in Mesopotamian Culture by Francesca Rochberg
  
• A History of Western Astrology, Vol. I: The Ancient and Classical Worlds by Nicholas Campion
  
• A History of Western Astrology, Vol. II: The Medieval and Modern Worlds by Nicholas Campion

If you're reading books from the 900s, you're reading THE resources (depending on who you're reading). However, they're not all that easy to read, for sure.

I highly recommend picking up a couple books on the history of astrology, such as A History of Western Astrology by Nicholas Campion. Also, a recent release by Chris Brennan is basically a huge Hellenistic astrology handbook, and it also covers some history and philosophy. If you're looking for something smaller or more introductory, there's a small intro to traditional astrology book by Ben Dykes that's excellent.