Recently I found the compendium of design and development processes by Dubberly (How do you design?). It is the most comprehensive collection of design and development processes models in architecture, industrial design, mechanical engineering, and software development I have ever seen. The aim of this project was to bring together the practices and thinking about processes in other disciplines.
How designers think has been the subject of much research; one of the most valuable piece of work on this issue that I have encountered is Henryk Gedenryd’s (1998) doctoral thesis (How designers work – Making sense of authentic cognitive activity). And also Nigel Cross’s Book “Designerly way of knowing” is interesting to read in this context.