Text only emails are a reliable alternative to HTML emails. They work in any email client and can be read by all. Here are some tips on how to make text only emails more user friendly.
Just Ask is a detailed book on how to incorporate usability and accessibility into your design process. If you are considering conducting usability testing particularly for disabled users this is an essential book.
Neilsen delivers another brilliant book full of timely advice to help avoid common usability mistakes. A must read.