Blink is a book about “thinking without thinking” but really, it’s about how the vast majority of thinking works. The insight in this book is especially useful when dealing with people who aren’t particularly well equipped to discuss their subconscious reactions to something relatively subjective…like design.
100 Things Every Designer Needs to Know About People is a list of all the bits of psychology that they should be teaching in design school. It’s neatly organized, easy to refer to, and easily the best $30 I’ve spent on books in the last few years. If you’re even remotely interested in improving the effectiveness of your designs through an understanding of cognition, this is a great place to start.