Some time ago (in 1998), Eric Marsden wrote a series of articles on Emacs for Linux Gazette. While some of the material is dated, there's a lot of good stuff in there. Here they are:

• on jka-compr (AKA auto-compression-mode
• on networking, ange-ftp, w3, and crypt++
• an introduction to ediff
• on gnuserv
• on customizing Emacs, including material on Custom
• on abbrev, dabbrev, and completion