Well, one resource you will want to have a look at is the Encyclopaedia Iranica. I am not sure exactly how in-depth their 20'th century articles are, but at minimum they should point you toward further resources.

Secondly, you probably want to have a look at "All the Shah's Men" by Kinzer for a look at the background. It's quite a polemic work and not one you should use uncritically, but it's an easy read.

Another possible approach is to look at published translations of Khomeini's rhetoric (this is just one example) (assuming you're not fluent in Persian). Together with other sources you may be able to find an angle along the lines of, who did his rhetoric resonate with, and why? E.g. was there an urban-rural divide, a class division, ethnic divisions, something else?

That's all I can think of at the moment. I hope that's some help.

Oof, one more thing: I would like to disabuse you of the framing that it was about "resisting modernization". Khomeini's political ideology was the exact opposite: an Islamic ideology for the modern world.

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