Technically, Dave and Randy are both right about the scarcity of Duff-Gordon's autobiography. As Randy pointed out, during her time, the book went through many printings - so in theory there should be a large number of copies out there.
On the other hand, I follow the used book sites closely, and the reality is that a copy of the book only pops up a few times a year, on average. Where all those used copies are is anyone's guess, but in modern times, it is not an easy title to find.
all the best, Michael (TheManInBlack) T