All job-hunting correspondence is important, but the letter you send your friends is absolutely critical.
Here's why: When you're in a job-hunt you're selling personal services—what you can do—something intangible. People buy services based on trust. Marketing personal services is not like marketing a product. Shoppers buy products knowing they can return them if dissatisfied. But companies can't return employees who fail on the job. They have to terminate them and start over, both of which are costly. That's why employers are so cautious.
Crucial hiring decisions are generally made by a team. Key managers meet to define the duties and responsibilities and decide what sort of person they want. Then they ask, "Who do we know that could fill this slot?" Most of the time, someone in the group knows someone. That candidate is interviewed first, given preferential treatment, and usually hired.
The moral of the story is that managers hire their friends—known quantities, not shots-in-the-dark. No one likes to hire strangers—there's too much at stake. One wrong employment decision can ruin a manager's career.
What does this mean to you?
It means your next job is probably going to come from one of your personal friends or business acquaintances—or else from one of their friends. Not from a recruiter. Not from a newspaper ad. Not from knocking on doors or pounding the pavement.