The amount of time it takes for a book to be considered public domain in the US (typically 90 years, i.e. any book published before 1923) is 3.6 times longer than the amount of time it takes for the CIA to declassify a non-sensitive document (25 years, though sometimes even 10 years).

Even the freaking Pentagon Papers have been released into the public domain, while The Catcher in the Rye (1951) hasn’t.