
I also like Nigel Cross's stuff. His book Designerly Ways of Knowing (2006) is good but published in some absurdly expensive, for-libraries-only monograph series, so find it in a library rather than buying it. His more recent book, Design Thinking, is a collection of case studies with some analysis, a bit lighter but also good (and not $149: http://www.amazon.com/gp/product/1847886361/ref=as_li_ss_tl?...)
The book goes on to explain how the three disciplines differ as per the skills required to practice it: scientists need analytical thinking, designers need synthetic thinking.
You learn this by page 10. The rest of the book is just as interesting as its first chapter.
For the engineering-design side, two good books, imo:
Herb Simon's classic Sciences of the Artificial, which approaches design from the lens of an AI researcher trying to figure out what design really is, partly to develop it into a science of design, and partly with an eye towards formalizing a model of design that a computer could use: http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&...
Designerly Ways of Knowing by Nigel Cross, which positions design as a third kind of inquiry, neither fully science nor fully humanities, but a kind of constructive investigation of objects and their properties: http://www.amazon.com/gp/product/3764384840/ref=as_li_ss_tl?...
