The books I recommend are: Non-Designers Design Book by Robin Williams. It focuses on topics like Consistency, Alignment, Proximity, and Contrast. With these four rules, you can make 90% of your designs look good. It also explains briefly on what colors and typography to use. When you've firm grasp of the fundamentals, it would make sense to arm yourself with more theory.
Don't make me Think by Steve Krug. It teaches you to simplify and focus on the end-user.