The literature on HTML e-mail design, marketing, and implementation is pretty sparse. I've linked to what I could find here. If you find other resources, PLEASE tell me about them so I can share them with other readers.
D.C. Denison, et al., "Revealing the Weak Links of HTML-enabled E-Mail,"Web Review, 1/18/1998. Compares e-mail browsers, documents difficulties in creating HTML e-mail newsletters. http://webreview.com/pub/98/01/16/feature/
Edwin Wang, "How to Make HTML E-Mail,"OurQuest Online. Step by step information about making HTML e-mail from scratch, with some pointers and tricks. http://www.geocities.com/ResearchTriangle/4182/makehtmlmail.html
"HTML E-Mail Woes,"Woody's Office Watch, 12/4/1998. Complaints, but not much help in formatting HTML e-mail that will be seen consistently by various e-mail clients. http://www.zdnet.com/zdhelp/office_help/wow98/wow1204/1204_04.html
Carter Stowell, "All about HTML Email,"Web Monkey, 2/26/1998. A helpful technical article on the weaknesses, strengths, and tricks involved with HTML e-mail. http://hotwired.lycos.com/webmonkey/98/08/index3a.html
ZD Net AnchorDesk Change Format page is a great example of making it easy to select the appropriate version for your e-mail software. http://www.zdnet.com/anchordesk/whoiswe/changeformat.html