Over the years I've read a lot of design books and I have recommended many of them here. But a lot of people have ask me if I would recommend just one book (proving again that the more choices you offer the harder it is to decide; the paradox of choice). It's difficult to recommend just one book for the non-designer interested in becoming more design mindful. But if you are looking to take a first step in learning about the fundamentals of graphic design with an eye toward becoming a better judge of what is effective visual communication and what is not, then I suggest The Non-Designer's Design Book, 3rd Edition (208 pages) by Robin Williams. This book has sold a ton over the years and is highly recommended by just about everyone. It's not an overly comprehensive book, but I like it because it's small and focused on a just a few fundamentals that, if understood well, can indeed make a difference.
If you don't mind a book twice as thick, then you could get The Non-Designer's Design and Type Books, Deluxe Edition (448 pages) by Robin Williams. This is a combination of the Non-Designers Design book plus the Non-Designer's Type book.
A bit more meaty
If you want something a little bit more challenging, but still very much accessible and engaging, I suggest The Elements of Graphic Design: Space, Unity, Page Architecture, and Type (160 pages) by Alexander white. I like this book too because it's relatively small and easy to take with you as you travel. This is one of my favorite graphic design books and is a good complement to Williams' books on the basics. White goes into a bit more depth on space, unity, type, etc. and features a discussion on "the seven design components (including Gestalt, etc.) which I summarize here on my website.
Right, so I said I would suggest just one book and then went and mentioned two (or three, depending on how you're counting). Another great book is Universal Principles of Design (216 pages) by William Lidwell et al. I love this book but it's a larger hardcover. At some point you will want to get this one for your bookshelf. So, do you have a design-related book that you can recommend? In other words, if you could suggest only one book on graphic design, what would it be? What one book has been the most useful for you?