Useful email
Subject lines
Writing and Copy
Helpful free tips: Copyblogger.com
Email rendering across email clients
Checking how subject lines render: http://litmus.com/resources/subject-line-checker
Also some nice easy online tools: http://www.emailonacid.com/
Statistics
Sherpa provides insight into most popularly tracked stats: http://www.marketingsherpa.com/article.php?ident=31781
Deliverability
Email on acid again: http://www.emailonacid.com/