A little different, if you haven't read either of the previous volumes in Sales' quartet, because his mix of alternate history, technological possibility and human endeavour IS refreshingly new; but also if you have, because here he focusses more on the human (and on real people).
The book is divided into six threads, named after the six quarks: up/ down, strange/ charm and top/ bottom.
"Up" and "Down" deal with two slightly different (I think) alternate realities - an Earth where the Korean war went on and on, so that the US had no men to spare as astronauts for the Mercury, Gemini and Apollo missions, sending women into space instead, and another where a deep sea expedition is mounted to recover film from a spy satellite. In both, unlike in Sales' earlier books Adrift on the Sea of Rains and The Eye With Which The Universe Beholds Itself the technology is very much as it really was, with the deviation being the history - especially the 1960s-female-astronaut thread "Up" (of course the deep-sea adventure is "down"). We see this through the eyes of Geraldyne Cobb, a real person, as were her 12 colleagues - who actually trained for missions that, in the Mad men era of the 60s, never came. But what if, what if things had only been slightly different? Then we see the possibilities - we see Cobb and the others having by no means an easy time, still the victims of sexist thinking and attitudes, but actually making it into orbit. It's a wonderfully imagined piece and I only wished that Sales had explored it in a more leisurely way - perhaps in a full length novel. In comparison, "Down" doesn't enthral quite so much, though it, also, perhaps has hooks to a potential wider story as Lt Cmdr McIntyre encounters the unexpected deep below the Bermuda Triangle...
The central two parts of the book - "Strange" and "Charm" - are shorter and in some ways Codas to the stories in "Up" and "Down". Sales doesn't wrap things up neatly but shows us where his stories are heading (this is similar to the treatment in the first two books of the Quartet.)
The final section - consisting of "Top" and "Bottom" - gives factual background to the earlier parts. This is something that sales' earlier books, which were heavier on technical jargon and acronyms, dealt with through appendices and lists of terms. Gives the more human focus of this book, it seems more appropriate that this is done in a more narrative way.
I enjoyed this part of the Quartet. It is a short book, and doesn't pretend to be anything else - but felt complete to me. Like a precious length of film retrieved from the deeps, you can see a series of pictures, but you can only speculate what was on the bits that never made it to the surface. Perhaps one day Sales will retrieve some more. I hope he does. In the meantime, we have these slightly disturbing Cold War dreams to show what might have been.