Write in a friendly, self-confident tone. Bypass the cutesy and over-familiar. Be Professional.
List any qualifications, relevant previous publication credits, writing credentials, especially if you've studied with a prominent person, or on-the-job experience that would lend credence to this particular manuscript or even what inspired the writing of this book.
Follow submission requirements to the letter. Your synopsis should provide a thumbnail sketch of who, what, where, when and why--the theme, setting, principal players, all in a few sentences.
The point of a cover letter is to introduce yourself and your work. It is the sample of your work that you include which will clinch the deal.