'Lockheed Blackbird - Beyond The Secret Missions'
I happen to be reading it at the moment. To be more accurate, it's been next to my bed for several weeks, and I dip in to read a few pages when I have time.... Didn't know it was appreciating though....
I was particularly interested in the early chapters about the A12, as this version is largely forgotten these days. I hadn't realised they'd built quite so many, or that they did quite so much work.