The book that came with the course - Writing and Selling Articles, Joyce White: This is a great book, with practical advice. It helped me when I started submitting articles.
Grammar For Smart People - Barry Tarshis: a good practical grammar book for solving problems or answering questions about punctuation. Because it is organized in a dictionary format, very easy to find what you are looking for, and good for problem-solving (when to use that, who, which for example) - it is the simplest and clearest book on grammar usage I have found).
The Art and Craft of Feature Writing -William Blundell. Another practical book.
How To Tell a Story: The Secrets of Writing Captivating Tales - Peter Rubie and Gary Provost. Good book when writing fiction.
Canadian Press Style Book - answers questions about usage: although oriented for journalists, it is great for solving problems of style and usage.
Finally, Sense of style by steven Pinker -is an academic book that contains really great advice.
Hope this helps
These are great suggestions! I have made note of them.