Dear /r/AskHistorians,

I am writing a history paper titled "To what extent was Alexander the Great's generalship the main reason for the victory at the Battle of Gaugamela?".

Now, I am not looking to have this question answered - I just want to know the best sources that I can use to answer it myself. I know that Arrian is considered the most reliable of the Ancient sources - but that he wildly over exaggerates the size of the Persian army as well as their training, and that more modern historians - such as Hans Delbruck - have drafted up more accurate statistics.

However, I'm not sure, for example, how much of Delbruck's book contains the actual battle, or how much of Arrian's or Plutarch's writings do. if you could help me with this, it would be greatly appreciated!