The absolutely massive transformation of Iran practically overnight in the 600 to 800 AD era continues to surprise me, despite being from around that region. Can you recommend books / documentaries from the perspective of the Iranian people about how and why this change happened? I want to get more insight into why such a large and proud empire changed so radically in such a short period of time. I am largely familiar with the Arab / Muslim telling of the story.

The story goes: Islamic tribes began conquering the Persian empire. As they went, the population was given two options:

1. Convert.

2. Die.

3. (Option which wasn't provided) Run away.

FWIW, if you want more info about why your parent commenter seeks a "comeback" is likely related to how I call Zorastrian ideas "infectious" in another of my posts on this thread. Check it out. Let me know what you think.

Also the Persian Empire was recovering at the time (poorly due to a succession crisis) from a rather long and brutal war with the Eastern Roman Empire.

There were seven great houses in the Sassanid empire and in the last years of the empire, they had been in conflict with the Sassanid court in South West Iran due to, according to some historians, years of overtaxing to support the war against Rome. Concurrent with the war, there was a plague in Western Persia killing half the population in 628.

When the Muslims invaded, the two most powerful houses refused to fight, and returned to their land in north east Iran. This left a weakened Western Persia alone to fight the invasion, which it could not successfully do.

my suggestion would be to have a look at "Two centuries of Silence" by Abdol Hossein Zarinkoob and "Sassanid Persia" by Arthur Emanuel Christensen.