Universal Principles of Design
William Lidwell, Kritina Holden, Jill Butler
A browsable encyclopedia of design principles, one concept per two-page spread.
This is the fastest way to build a shared vocabulary of design principles, from Gestalt grouping to the aesthetic-usability effect, each explained on a single spread with an example. Do not read it cover to cover; keep it nearby and look concepts up as they come up in real work. A strong starting point for someone new to visual design.