The Non-Designer's Design Book has been a great help to me. I bought copies for my employees because it was easy to read and showed practical examples of how a few basic principals can radically improve your designs.
Does Amazon auto add referral codes if you're logged in? I just popped over to their site, searched for the book and copied the URL. I won't turn away a commission but it wasn't intentional.
http://www.amazon.com/Non-Designers-Design-Edition-Designers...