I think it's important NOT to fishing for a job during the shadow and prep for the shadow. Although you may think it's a great time to slip me your resume - that is better for a follow up letter.
Just state where you are in your career, and what you hope to get out of the shadowing process. Most SM's are either pro-shadow (and will do it right away, so why bother writing a long letter) or anti-shadow (in which case a long letter will just annoy them).