The trick with using letters to publicize your work is using a low-key approach to incorporate information into a letter, while keeping the letter relevant to an issue. This is not that complicated, if you remember one cardinal rule:

See yourself as providing information, and not selling your work.

In other words, write as though you had no interest in people buying your services. Write as though that was not even a factor at all. Simply write as if you were just providing helpful information, and the sales will come.

The difficulty is that hard-sell might have worked at some point. Maybe the ancient Egyptians were sold Pyramid condos by hard-sell methods. Maybe. But people won