fathom is an organization that strives to define the intangibles of design. They have a good recommended reading list for delving into how and why we design. It is a good resource, but by no means comprehensive, for finding books relating to the psychology of design.
One of my favorites on the list is: Design Like You Give a Damn: Architectural Responses to Humanitarian Crises. The author of another book I reference frequently, The Design of Everyday Things by Donald Norman, also has a book on this list.
