My brother-in-law's book, "The Constant Contact Guide to Email Marketing," is doing quite well on Amazon. After the early spam-crazy days, email marketing has really had to rebuild itself from the ground up. I am a big believer in it, and can highly recommend his company Constant Contact as a email service. I have several accounts and have set most of the non-profits I work with on it. In his book, Eric discusses email marketing in the context of both customer acquisition and loyalty. With Google clicks going for $2 or more, email remains a great value if done right.