I'll second the suggestion of Kuhrt's "The Persian Empire" as a good starting point. You'll probably also be interested in Pierre Briant's "Darius in the Shadow of Alexander."

That said you're running into one of the core problems of studying the Achaemenid Persians, especially their later kings:

• The Persians of that time were not really a literary culture in the same way as their Greek or Judean contemporaries. They simply did not produce the same sort of written detailed histories internally.

• The records we do have are largely administrative and monuments. These are very useful, but not as direct or detailed.

• By the end of the 5th Century BC, most of the empire had largely transitioned to papyrus and leather as the writing media of choice over clay tablets. This was great for efficiency but terrible for long term preservation.

• The King's Peace at the end of the Corinthian War and Persia's subsequent focus on reconquering rebel territories like Egypt greatly reduced the amount of military and political interaction with Greece at the same time that Philip II was on the rise. So Greek sources in general were much more focused on their own affairs in that period.

• Later generations were more interested in Alexander. So when deciding which manuscripts to copy and preserve, Alexander got preferential treatment.

• In Darius III's specific case, there just plain isn't much of his reign that was not wrapped up in Alexander's invasion. He took the throne in 336, and Alexander invaded just two years later.