I picked up a design book by Mollerup called Collapsible. It's a survey of various approaches to creating objects that save space in novel ways. Saving space is always of interest to people who can't seem to find a bag to put all their junk in that they're also willing to carry around. I was on a bag-a-month plan for a while. I seem to have settled on a messenger's-style as the right form factor. The one that I carry when I can remember is by Eastern Mountain Sports.