Purchasing my copy of grid Systems in Graphic Design, by Josef Muller-Brockmann, was the best money I've ever spent on a design text. I was a relatively seasoned designer with 6 years of pro-experience when I was fortunate enough to find a copy of this masterpiece. I was largely self-taught however, and while I knew from working in various layout programs about grids, I had no idea how powerfully they could be utilized. Within a weekend of reading and absorbing this book, my designs had improved substantially. The grid theory and aesthetic principles set forth here are easy to understand and immediately put into use. The book lays bare the absolute essentials for successful corporate design, and the examples of the author's work from 50 years ago (the first IBM Corporate Identity) are so "current" you will be amazed at how little good, clean design principles have changed over the decades. Buy this book, wherever you find it... especially if Corporate Design is your thing.
For the novice, this book is an excellent introduction to the purpose and use of grid systems. For the seasoned pro, it offers interesting insights into the small details a grid system can emphasize, or harm, w/any design project.