cognitive psychology. PhD 
- Mind in Motion (Tversky) - Things That Make Us Smart (Norman) - A History of Mathematical Notations (Cajori) - Cognitive Dimensions of Notation (Green) - Changing Minds (diSessa)
- Cognition in the Wild (Hutchins) - The Nature of External Representations in Problem Solving (Zhang) I think cognitive psychology, representation theory, semiotics, and distributed cognition form the core of tools-for-thought. We know a lot more about them than e.g. education.
