For some not so heavy reading for SR-71 enthusiasts, I highly recommend Brian Shul's book "Sled Driver". They are out of print now I believe, and I've seen them on eBay for about $3000 at one stage, but I think they are more generally available at reasonable prices now second hand. I was lucky enough to be given a copy as a gift by a friend a couple of decades ago.

Beautiful book with amazing pictures and stories by a long time SR-71 pilot. A lot of the 'urban legend' stories going around about the Blackbird come from that book.

"Skunk Works", as mentioned by other posters here is also a good read.