Check out The Non-Designers Design Book by Robin Williams. It focuses on four fundamental concepts of graphic design that apply to everything visual, including

Check out The Non-Designers Design Book by Robin Williams. It focuses on four fundamental concepts of graphic design that apply to everything visual, including UIs. Its four concepts form a language for thinking about the aspects of visual design like color schemes, font sizes, spacing, etc. It has lots of examples and mini design exercises. And you can read it in just a few hours.