The Cover Letter: Door Opener Par Excellence
http://sciencecareers.sciencemag.org/career_development/previous_issues/articles/1890__1/the_cover_letter_door_opener_par_excellence

Jensen, David G.
Science
2002

Abstract:

Although we are allowed to put more into a cover letter than can appear on a magazine cover, the challenge is still to keep it succinct. In fact, writing something that is powerful and yet short is the single most difficult kind of business writing. You already know that although it's easy to go on and on in a company memorandum, saying the same thing in half the space can make your work twice as powerful.