I was intrigued by the film "Helvetica" and decided to try one of MIT's free online courses http://tiny.cc/a4Xr0 "Fundamentals of Computational Media Design." This is the textbook. It's fascinating.
A classic to be read and re-read. If you like this and are interested in the larger scope of graphic design, look for Megg's History of Graphic Design - the granddaddy textbook of graphic design.
I bought this book for a class on typography in college. It gives all the basic rules and information about typography which is very helpful for graphic designers or anyone else who wants to learn more about typography. Great book for reference!