Still the best book for contemplating a career change: What Color is Your Parachute.
I bought the actual physical book (I know!) because there are forms to fill out within the book, and it was just easier to buy the damn book then faff about with printing and whatever.
After I got the book, however, I saw that there now is an iPhone app. I still think I prefer the physical book format, but I am tempted to try out the app…