In the early 1960s, Chan Thomas wrote a book titled The Adam and Eve Story and attempted to have it published by Emerson House in Los Angeles. Soon after publishing the full issue, the CIA classified his book for 50 years. An FOIA lawsuit was filed against the CIA to release the book. In 2013, the CIA released a "cleansed" version of the original book (50 pages out of 240 original pages).
Why was this book classified?
https://www.cia.gov/readingroom/docs/CIA-RDP79B00752A000300070001-8.pdf
Which aspect? I can recommend quite a great deal of resources for you to examine. Just be a little more specific in the request and I would be glad to. I linked two channels earlier in the thread that are directly related to this with hundreds of hours of lecture and information.
But what is it, specifically, you are asking about?