I had a meeting today with a PhD student from another department and was asked what to read if you want to get into the more theoretical and philosophical aspects of design and that had influenced my work. It was a good exercise and I came at least up with a few books, even though I am sure I have forgotten some that might be even more influential on my thinking. So, here they are, not full references but some with links:
Lundeqvist, Jerker. (1982?) "Norm och modell" (Norm and Model). in Swedish
This was Jerker's PhD thesis and it really opened the door for me to design theory!
Simon, Herbert. "The Science of the Artificial"
Simon's book is a must in the area.
Schön, Donald. "Educating the Reflective Practitioner"
Maybe the most influential book for me.
Lawson, Bryan. "How Designers Think"
Lawson's book really helped me to form my own understanding of design.
Dunne, Joseph "Back to the Rough Ground"
Probably the best book ever on practical knowledge and judgment.
Krippendorff, Klaus. "The Semantic Turn -- a new foundation for design"
The best contemporary book on design theory.
and of course the "best" one :-)
Nelson, Harold & Stolterman, Erik. "The Design Way - Intentional Change in an Unpredictable World"
Well, I will maybe update and add more later.....