I am not sure why this was classified. It might be because it was the material was from the files of some higher-up for whom they classified everything they didn’t know what to do with. There is some apparently unrelated material, handwritten notes and three pages from time magazine, etc., which seem to have been stuffed into parts of the book and were reproduced where they were found Some parts of the handwritten notes have been blanked out. Some pages of the book following section titles were not reproduced, probably because they were blank.
There is an Amazon listing for the book but it says “out of print, limited avaiability” and lists no sellers.
Here is a link for the PDF of the CIA copy which was declassified in 2013, presented for your discernment:
Here is a page from the book listing inter-cataclysmic periods: