Written by William Lidwell, Kritina Holden, Jill Butler.
This book was one of the first and most important books I have read learning design. It has learned me what problems design can solve, what the best practices are for many fields of use and gave great insight in the value it can provide for clients. There also are a lot of base principles that are very well explained, one of which is the golden ratio.
Most of the information that is written in this book I apply in any of my design and branding work. As it makes you understand that design is not art, it is always used as either a way of communicating to a specific target audience, helps to achieve a certain goal or solve a major problem.