Designers probably will get most out of this book, but even without designer skills, there are valuable tips and tricks on improving the visuals on your slides.
I would recommend Beyond Bullet Points: Using Microsoft® Office PowerPoint® 2007 to Create Presentations That Inform, Motivate, and Inspire
to get a better idea on the concept of creating text-light slides. Content is king, so get this one first.
I'm not thrilled by some of the graphs slide:ology features. Other than that, I got a lot out of it.