I bought a really interesting book about the subject a few months ago, essentially a history of the CIA's Office of Technical Services, 'Spycraft' by Melton, Schlesinger and Wallace. It goes into some detail (but not too technical unfortunately for us) about the items used during the cold war and how they were inserted into walls, rats, tables etc etc. The transition from valves to transistors was the big game changer evidently. I really recommend it.
Andrew
https://www.amazon.co.uk/Spycraft-H-...=UTF8&qid=&sr=