Conceptual Models: Core to Good Design. User interfaces are simpler and more task-focused when designers start by constructing a conceptual model of the intended application. This book explains what conceptual models are, how to construct them, and why they are valuable.
Designing with the Mind in Mind: Simple Guide to Understanding User Interface Design Guidelines (2nd ed.). An introduction to cognitive and perceptual psychology for software designers and developers. Now in its 2nd edition.