I was mostly interested if there are any good sources or readings about Achaemenid Eunuchs? I'm guessing not too much info exists, but I was intrigued after I had learned of Bagoas (the Bagoas who supposedly served both Darius and Alexander the great).
I think I read somewhere that they may have adopted the practice from the Assyrians or Babylonians. and I would also be interested in any further readings or knowledge about them!
I'll echo /u/Trevor_Culley that there is an odd bit of dead zone for that place and time period and eunucholgy. I can definitely do you Hittites, Assyrians, Greeks, Romans, Biblical etc.
However! You may be excited to know that there is an open access article specifically about Bagoas from the incomparable Dr. Tougher:
It's primarily literary analysis, but Tougher is the leading scholar in ancient-times eunuchs, and I think you'd like it.
I also think you'd like this:
I'll dump you my Zotero folder of "Eunuchs in the Ancient and Classical World" into a pdf for if you're very keen... Not sure how deep you want to go and how much you want to test the ILL department!
There's actually surprisingly little written about this. Partially because there just isn't much evidence of them one way or the other. There are some words that might refer to eunuchs in the Persepolis Archive Tablets, and Greco-Roman authors reference Persian eunuchs as a matter of course, but apparently did not think their audiences needed much explanation of the practice. That said, I do have a few suggestions.
As with almost any topic in Iranian/Persian history, Encyclopaedia Iranica is a great, free place to start. Specifically "Eunuchs I. The Achaemenid Period." It's a short article, but does a good job of pulling together the scant information we have in one place. If I were trying to write an answer for this sub about Achaemenid Eunuchs, I'd probably just end up reinventing the wheel with this one.
Then there are two books. Neither of these are explicitly about Achaemenid Eunuchs, but both have chapters on them and reference them in relation to other cultures.
Though not explicitly about Eunuchs, I'll also recommend King and Court in Ancient Persia: 559-331 BCE by Lloyd Llewellyn-Jones. He also wrote the chapter on Achaemenid eunuchs in the first book I listed above, and they feature prominently in his explanation of Achaemenid court life.