MarketingSherpa is offering a free download, Dirty Dozen: Email Newsletter Mistakes Nearly Everyone Makes. Find it under Email Marketing Resources on this page, along with other free downloads.
My favorite mistakes (which I see all the time): Lame 'Welcome' Messages [see my ClickZ article from October 2001: Leveraging Your Confirmation Emails] and Ignoring Your Blackberry and Mobile Readers.
The free guide is but a small taste of what they offer in their annual door-stopper (328 pages): MarketingSherpa Email Marketing 2008 Benchmark Guide. I've reviewed these annual mega guides in the past (see 2006 review). They're chock full of charts and stats and really helpful if you're looking for metrics to gauge the success of your email programs.
P.S. Full disclosure
Yes, I'm a MarketingSherpa affiliate and receive a small commission if you order through my link above.