A while ago, NRO put on-line a bunch of declassified documents [1], including their internal books about the history of satellite reconnaissance. But it was all in one unsorted pile, often scans of barely legible pages. I think this was the original source of some of the documents shown in this article.

Although one could randomly stumble on some interesting anecdote here and there in the NRO archive, overall that collection was not made to be easy to navigate, and one would have to be very patient to sift through it thoroughly. Maybe the situation is better now.

[1] https://www.nro.gov/foia-home/foia-declassified-nro-programs...