Resumes and Cover Letters
The impact of job search correspondence cannot be understated. How you represent yourself on "paper" has a tremendous impact on your chances for being selected for an internship or job. An effective resume and cover letter are likely to form a positive first impression to an employer, resulting in a competitive advantage.
Other Letters
All correspondence develops an impression; even a brief email makes an impact. A timely and well-organized thank you letter following an interview can dramatically sway the employer's deliberations in your favor.