
To learn about e-mail marketing in general:
E-mail Standards Project: http://www.email-standards.org/
Mail Chimp's Fabulous Guide on HTML Emails: http://www.mailchimp.com/resources/email_marketing_guide.phtml
For help with the coding process:
A comprehensive article on coding e-mails: http://www.sitepoint.com/article/code-html-email-newsletters
A Guide to CSS Support in Email: http://www.campaignmonitor.com/css/
W3Schools Online Tutorials (a helpful resource if you need to look up code): http://www.w3schools.com/default.asp
A great article on working with tables in your HTML e-mail design: Tables in HTML Emails: Nesting, Padding and Widths, Oh My
20 HTML Email Tips: Ignore at Your Own Risk from Email Labs
E-mail marketing companies (for use in sending your marketing emails):
Constant Contact
Mail Chimp
Campaign Monitor
This should be a good start for anyone looking to delve into the world of e-mail marketing. Please leave me a response if there is anything else you would find helpful on the subject.