Bruce "Tog" Tognazzini is one of Interaction Design's elder statesmen, best known for designing Apple's early interfaces and writing the Apple's first Human Interface Guidelines. He first posted his "First Principles of Interaction Design" to his website in 1999, and he’s updated them every few years ever since. He recently posted his latest "Revised and Expanded" version, which includes major updates. Definitely worth a read.

Bruce Tognazzini’s First Principles of Interaction Design (Revised and Expanded)