I learned 2 out of 3 of these (possibly I forgot #3) from a kid book in 'how to become a spy' which I borrowed from the local library. I wasn't into literature but into informational books. I was eating all these books like crazy. There were loads of tricks in this one, including writing with milk and using lemon to make it appear. Shoe lace tricks were also there. Techniques on how to follow someone. This was at the very end of Cold War, begin 90s.
There's good podcasts out there about PIs, spies, military intelligence, and good ol' police work. So I still love the content. The last one I finished was yesterday, about the murder of Dutch politician Pim Fortuyn. The murderer, who was pretty much caught red handed, was curiously living next to a former top criminal which was omitted from the police report to protect this person against public outcry. This person a few years later happened to live near an Islamic terrorist. So the question arose if the former top criminal gave the weapon to the murderer, or whether he was an informant for the AIVD (back then BVD). Of course, the former top criminal passed away whilst the podcast was being made (classic plot twist). Regardless, a fascinating story, albeit inconclusive.