I read Burn After Reading: The Espionage History of World War II by Ladislas Farago. I’d hoped it would reveal something useful for Kryptos but it has not.
I would, however, recommend it to anyone interested in WWII, spies, or cryptology. The sheer amount of detail is staggering down to the individual names of spies that only spied a couple days. It’s not comprehensive enough to hurt the brain but it does a very good job of covering the whole of World War II in the context of the intelligence communities and activities of the nations involved.
Another thing it will communicate is the depth of espionage activities that lie outside of actual codes. While espionage relies quite heavily on secure communications it is the context of 20th century cryptology that will help a 21st century hobbyist in their own efforts.
I highly recommend this book.