"The Non-Designer's Design Book" by Robin Williams is the best introductory book I've read. As the title suggests, it covers the fundamentals - proximity, alignment, repetition and contrast - in a really approachable way.
It probably won't teach you to make a great design, but it will prevent you from making a bad one.
Actually, they're presented in the book as Contrast, Repetition, Alignment, and Proximity.
Williams jokes in the early chapters that she wanted to find an easy-to-remember acronym for the four central concepts of the book - and boy did she pick a good one.
It probably won't teach you to make a great design, but it will prevent you from making a bad one.