73 rules of spycraft by Allen Dulles

it might be useful; it might be the opposite; it may be of no consequence whatsoever
looks draw looks
"An insipid letter is in itself suspicious"
do suspicious things after dark or on early mornings, when people are half-asleep and not on the lookout for suspicious dealings
for interviews walk outside -- cemeteries, museums and churches; walk in the countryside
bring reading matter to avoid conversations or break off an embarassing one
easiness and confidence should be learned and practiced; they help us personally and they help produce similar reactions to those we are handling
it often takes more moral courage to ask someone to do something dangerous that to do it yourself, but if this is the proper course of action you must follow it
if you have an agent who is really important to you, try not to let him know this
once people label an agent, the job is half done (about covers). People take things so much for granted and only with difficulty change change their sizing-up of a man once they have it (Also mentioned in "Psychology of intelligence analysis" -- changing a view needs much more information that /directly contradicts/ it than forming a view
when your name creeps up in conversation, the people need to be able to say something about you, different from your real work
never know too much among your other contacts;bite down on your vanity, which would like to show how much you know. Especially hard when you hear a wrong assertion being made
within the limits of your principles, be all things to all men, but don't betray your principles. The strongest force in your show is you.
it's your job to bend the circumstances to your will, not the other way around.

As mentioned in "Hacker opsec required reading" @ https://grugq.github.io/blog/2013/11/06/required-reading/