If you want to understand the psychology of design you need to look for books on "human factors" and not web design books. Human Factors is a scientific discipline that is a subset of IO psychology.

While many of the books recommended on this board are good books, some of the authors learned through trial and error without the guidance of the scientific rigor that is provided by Human Factors, while others are just plain wrong.

The mind works in particular patterns, understanding those patterns will help you conceive of interfaces and test those conceptions, while verifying that it does indeed improve whatever metric you are trying to improve, not just emulate something that worked for someone.

Now I am not saying you need a PHD in Human Factors to design good interfaces, but a little grounding goes a long way. It will help you understand why something worked for someone in a book and determine if it will also work for what you are trying to apply it to.