Gopen & Swan: The Science of Scientific Writing. A great article with useful advice applicable to any kind of writing. Read it several times — preferably before taking the class. George Gopen has also written a great book The Sense of Structure – Writing from the Reader´s Perspective.
Barbara Minto: The Pyramid Principle. The classic book introducing the pyramid principle – the gold standard for business writing.
Strunk & White: The Elements of Style. The short book about writing. Sold in more than ten million copies.
Peter Drucker: Managing Oneself. A classic from Harvard Business Review which talks about different learning styles.