I recommend the following RFCs.
The Section RFCs and Internet Drafts
explains how to get them.
You can also click on a link to read from a central Web server, but finding
another site with copies will probably give you faster response.
Here are some other useful publications and books:
Anderson, Robert H., Norman Z. Shapiro, Tora K. Bikson, and Phyllis H. Kantar,
The Design of the MH Mail System,
The RAND Corporation, Santa Monica (CA), December 1989.
(N-3017-IRIS)
Garfinkel, Simson,
PGP: Pretty Good Privacy,
ISBN 1-56592-098-8,
O'Reilly & Associates, Inc., Sebastopol (CA), 1994.
Hahn, Harley,
A Student's Guide to Unix,
ISBN 0-07-025511-3,
McGraw-Hill, Inc, New York, 1993.
(Not just for students.
An excellent guide to UNIX.)
Kernighan, Brian, and Rob Pike,
The UNIX Programming Environment,
ISBN 0-13-937681-X,
Prentice-Hall, Inc., Englewood Cliffs (NJ), 1984.
Kochan, Stephen G., and Patrick H. Wood,
UNIX Shell Programming,
Hayden Books, Indianapolis (IN), 1985.
(A revised edition is out, but it unfortunately doesn't cover the C Shell.)
Ousterhout, John,
Tcl and the Tk Toolkit,
ISBN 0-201-63337-X,
Addison-Wesley.
(By the creator of Tcl and Tk.)
Peek, Jerry, Mike Loukides, Tim O'Reilly, et al.,
UNIX Power Tools,
ISBN 0-679-79073-X,
O'Reilly & Associates/Random House, fourth printing May 1994.
Rose, Marshall T.,
The Internet Message: Closing the Book with Electronic Mail,
ISBN 0-13-092941-7,
PTR Prentice Hall, Englewood Cliffs (NJ), 1993.
Rose, M. T., "The RAND MH Message Handling System:
The UCI BBoards Facility."
(This paper is distributed with MH.)
Wall, Larry, and Randal L. Schwartz,
Programming perl,
ISBN 0-937175-64-1,
O'Reilly & Associates, Inc., Sebastopol (CA), January 1991.
Welch, Brent B.,
Practical Programming in Tcl and Tk,
ISBN 0-13-182007-9,
Prentice-Hall, 1995.
(By the author of exmh and this book's
chapters about it.)