Most of these are very old, and things have probably changed since they were written. Also, back then the community was smaller and information was not as readily available, so some things might not be best practice. However, (some) things were also simpler, and these documents might provide a blueprint for accomplishing the same goals with more modern technology. Also, knowing history is always good.
These are my personal picks.
Encryption:
Mail:
sendmail. Don’t use sendmail.)