The Architecture of Happiness by Alain De Botton.
Sometimes, someone lends you a book, and it so grabs you that you immediately throw everything else to one side, and finish that book, regardless of everything else you are reading. This was one such book.
Mostly about architecture, and a little about art and human perception, this was a book that tried to explain what it is about our environment that so affects our mood. Botton speaks eloquently about the possible reasons for a particular culture finding beauty in one style, but not another. An eye-opening read, and filled with photographs of the various works and buildings that it examines. Well worth a look if your job (or your hobby) is in any way visual.