There are many books on design both print and web. Below are a list of reference books I suggest that will help you not only in design but in business:
Designers in Handcuffs - Pat Matson Knapp
Idea Revolution - Clare Warmke
Pantone Guide to Communicating with Color - Leatrice Eiseman
Experimental Typography - Rob Carter
Man and His Symbol - Carl Jung
Even though the last one is a psychology book I recommend it, because it helps you understand symbolism more. This relates to how color, imagery and type effect your audience.