I had long pined to read Tufte's The Cognitive Style of PowerPoint, but couldn't convince myself to spend the money; when I arrived at university, I immediately found a copy, and greatly appreciated it (not to say I agreed with every point). I thence read Visual Display of Quantitative Information, and found it fascinating--for anyone who's ever taken an interest in typography, visualization, or the theory of communication, it's an excellent read. Even if none of that interests you, you'd still benefit from the book: it shows you how to think analytically about human factors, which is a relatively rare discipline.
Also recommended in this reply