Naomi Karten is a wonderful author, speaker and consultant for organizations trying to improve their customer service. Among her specialties are creation of effective service level agreements.
Roger von Oech has written a variety of books about problem solving and creative thinking.
Jerry Weinberg and Don Gauss wrote a book called “Are your lights on?” in 1982 that was very influential in our thinking and consulting about problem solving. Particularly interesting was their notion of the Definition of a Problem: “A problem is a difference between things as desired and things as perceived.”
Edward de Bono is one of the few authors to explore tactics for problem solving.