Exile and the Nation by Afshin Marashi is a recent book about the interactions between Parsis and Iranians in the 20th century
https://www.amazon.com/dp/1477320792/ref=cm_sw_r_awdo_RCTXFJEEWDB8XTXWP0EG
I would start with the Gathas of Zarathustra. There are free translations available online. There is also a very good translation available on Amazon, I recommend this:
There are a lot of reprints of the original Sacred Books of the East series, which is still the only complete translation of the Avesta - as well as many of the Pahlavi texts.
I think this reprinting is best: https://www.amazon.com/Sacred-Books-East-Zend-Avesta-Part/dp/1402185898/ref=sr_1_17?dchild=1&keywords=avesta&qid=1597774481&sr=8-17
(that's just one of the volumes, but you can navigate to the others in the recommended links)
I like this one, because another prominent series (Forgotten Books) have retyped the whole thing into a different font that doesn't represent the original diacritics or footnotes accurately. This reprint is just a bound photocopy of the original.