This is an important book, moving interaction design from an art form to a science. Harold Thimbleby provides a treasure trove of information, introducing powerful methods, yet so skillfully explained that even the non-technical reader can follow and learn. The methods promise to make dramatic improvements in the design and evaluation of interactive systems. Don’t worry: the formal tools are introduced slowly, step-by-step. Even if you normally fear such methods, you will be entranced and enchanted. Buy this book and use it. I know I will.