Books on Design Theory or Principles?

Elements of Design was the fundamental book on this while I was a student.
It does approach design mostly from a designerly aesthetics perspective while a design also needs to make sense as a business sales point, fit into the lives of the people in your target market, and into current-day manufacturing processes. What Michael outlined is a great process, for more in-depth understanding read a few papers by Pieter Desmet or about Kansei Engineering.