An ebook that was/is available for purchase in one go was distributed for free a section at a time.
Most prespectives on email marketing is about the ROI of the email campaign and many sales people and consultants really struggle with the publishing angle. Clickz itself is a good example as well as Email Marketing Reports.
Providing free content online and via email then wrapping it with relative adds gives the audience what they want without interupting them with ads to get to the content. At the same time there is ample space to advertise products and content of the same subject to people who have already show interest in it.
This can be and is expanded into newsletters and some transactional emails to add additional cross and up sell opportunities within the same brand.
If viral tracking is implemented properly, incentives can also be provided for current subscribers and customers to encourage them to recommend leads and prospects.
Leveraging and encouraging the people who interact with your brand the most to market for you is the way forward, especially with so many social media tools available now (Seth Godin).