The Design of the MH Mail System (1989) [pdf]

I still use this mailsystem albeit less than I did in the 80s and 90s. To me, it's the pinnacle of "it's just Unix files" as a model for information management.

(n)mh is actively maintained. A loving cast of characters from rand have been involved in email, including the much missed Norman Shapiro who wrote the RAND report on email misbehaviour.