Email

I consider email to be the most important communication program—or app if you must—ever written. I enjoy reading about how it came to be. Below are links I've compiled over the years, many to webpages written by the people who wrote the early email programs.

Webpage by Tom Van Vleck, one of the early authors of a program to send messages to other users of a time-shared computer.
May be the only reference you need as it has many links. Focused on email for time-share systems, i.e., not networked computers.
Did My Brother Invent E-mail with Tom Van Vleck?
Five part series by Errol Morris writing in the NY Times. This is part 5. The other parts may be linked from this webpage. Errol was Noel Morris's brother.
The Technical Development of Internet Email by Craig Partridge.
Detailed paper about email history.