An interesting read is Spy Catchers written by first science officer from MI5 in cold war. he was tasked with reversing listening devices and making them.. it doesnt spill all the beans but givea a good insight into how far they were already then.
u can hardly imagine what tech exists now 40-60 years later as electronics and computers have shrunk drastically and infinitely more weird material properties are discovered....
The Spy Catchers - The Official History of ASIO, 1949-1963 - https://www.amazon.com.au/Spy-Catchers-Official-History-1949...
The Protest Years: The Official History of ASIO, 1963-1975 - https://www.amazon.com.au/Protest-Years-Official-History-196...
The Secret Cold War: The Official History of ASIO, 1975-1989 - https://www.amazon.com.au/Secret-Cold-War-Official-1975-1989...
