Any graphic designer worth their salt should care as much about the “why” of design as they do about the “how.” To that end I highly recommend the great collection of theoretical essays, Graphic Design Theory edited by Helen Armstrong. The book is a fascinating collection of thought provoking essays by a wide range of designers including Steven Heller, Jan Tschichold, El Lissitzky, Paul Rand and dozens of others.
There is a LOT of great food for thought in this book, and it will take you on a long journey, highlighting many of the insights, arguments and controversies in graphic design over the last century. I particularly enjoyed seeing how design theory changes over time, and is influenced by the larger trains of thought that are happening in the wider culture. For example, notice how ideas of modernism and the machine turn up in the writings and theories of the Bauhaus then are challenged by later thinkers and designers.
For those who find theory boring, we’ll what can I say? You need to think about what you are doing or you are just tossing pixels and paint about. But for those who want to learn more, to dig deeper and to see how design fits into the larger cultural and artistic milieu then this book is a great starting point.