User Friendly: How the Hidden Rules of Design are Changing the Way We Live, Work & Play

The Design of Everyday Things, where he talks about the gulf between the system image (the interface) and the user’s model (the perception of the interface formed by the user through interaction with it).

Two of the most important characteristics of good design are discoverability and understanding. Discoverability: Is it possible to even figure out what actions are possible and where and how to perform them? Understanding: What does it all mean? How is the product supposed to be used? What do all the different controls and settings mean?