According to a 2011 Q3 Epsilon report released last December, emails focused on editorial (aka, blog) content received the highest click-through-rate and second-highest open-rate of any email message type:

This reinforces what smart marketers already know and practice, repurposing quality content across different marketing channels is a highly effective use of existing resources. Long-form content posted on your content marketing hub should also be utilized in email and social marketing, giving those channels additional depth and value.
I'm eagerly awaiting the release of the Q4 Epsilon report, but Compendium hasn't been sitting around doing nothing. We've built an integration with ExactTarget that makes that the creation of editorial content emails much simpler. With the click of a button, content in Compendium can be moved into your ExactTarget account and easily placed into an email. Integrations with several other major email service providers are also likely to be released this year.
If you're struggling with which kind of content to use in your email markeitng, try tips, stories or news. This type of content tends to be short, engaging and useful. Expecting a subscriber to check their email and then decide to read a 3,000 word essay is unrealisitic. Here are some good examples:
Daily Health Tip from Compendium client, Swanson Health Products
Cruise Stories from Princess Cruises
Pet Care Articles from Petco