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.
This is an excellent book that discusses human factors at a lot of levels from city planning, to the design of a simple device. It is kind of like a mash up between "Powers of Ten" and a Malcom Gladwell book. Not web centric, but a really good read and intro to HFE.
He is probably talking about the handbook of human factors and ergonomics. It has been updated since I purchased it, and is more information systems focused now. It is like the bible of usability but it is an expensive text, that is why I did not recommend it.
I guess I should have and put a disclaimer on it. When I bought it, it was around $275. If you have to buy one book and you can spare the change, this is the one. But some of the others will get you going faster.
It depends on what you want to do, This book is deeply focused and considered an academic text, many teach from it. Once the subject matter in this book is mastered you will be a HF professional for sure, but some of the other books can get you up and running faster.
At some point you should consider this book for your bookshelf but it is not a beginners book that is another reason I was hesitant to recommend it.
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.