This is the book to read if you want to understand the ethos of designers and understand design culture. Perfect way to begin to feel like you’re in the club.
The Elements of Graphic Design by Alexander White
By far the most organized thinking about design composition that I’ve encountered.
The Elements of Typographic Style by Robert Bringhurst
Not such breezy reading as the other two, but a very structured approach to typography, something that is normally left mostly to intuition.