Hi, sorry if this is inappropriate for this sub or anything, but I've always been fascinated with Iran. I have been reading mostly about the Achaemenid and Safavid periods. I tried scouring the internet and my university library for texts on the post-Alexander Seleucid period, up to the rise of Arsaces I and the re-establishment of an Iranian state, and later Ardeshir Babakan and the rise of the Sassanians.
Thanks for your help guys!
Arsacid (Parthian) history is very sparse, due to the limited amount of material. The closest thing I even own is Arsacids & Sasanians by MR Shayegan, and it is an immensely dry and tedious quest in teasing political ideology out of philology. Some useful digital resources are http://parthiansources.com and http://iranicaonline.org . Other than that, peruse jstor, academia.edu, your local university library database, etc for papers, because to my knowledge there isn't really a definitive history of the Parthians.
The Sasanians are easier: Touraj Daryaee is your go-to author. I like his textbook-style Sasanian Persia, but I've talked to professors who suggest his From Oxus to Euphrates as the best introduction. Encyclopaedia Iranica and academic papers are great to perude in this case too of course.
I hope that helps you!