Learn the best practices in newsletter designs, use of images, subject lines, personalization. Also include topics such as statistics, HTML coding, etc.
Premailer does the nasty work of converting good HTML to the awful dialect used by common e-mail clients.
Jakob Nielsen's classic article on newsletter design usability.
Comparisons of good and bad subject lines — from Mailchimp.
Be careful with images in newsletters.
Another excellent Nielsen article, this on on the Subscribe/Unsubscribe Process and the importance of presonalization in newsletters.
How HTML email works, basic concepts, best practices, tips and tricks.
Take this with a healthy dose of skepticism. I suspect that webmail clients are far more prominent, but there are no good statistics.