First paragraph, introduce yourself. Second paragraph, introduce your work. Third paragraph, add anything important you left out before, like potential markets. Fourth paragraph, say goodbye and get out. Keep it to one page.
Editors are busy people.
Write every day, even if it's only one paragraph a day. That's one paragraph more than you had the day before, and by the end of the month you might have a chapter. If you don't use your skills, you'll lose them, and then you'll truly never get published. And keep your stuff in the mail, not on your desk. It won't sell from your desk.