This excellent book is alternative history in space with a difference: it it begins in 1988 on Venus in a world where, as in the Martian fantasies of Edgar Rice Burroughs or other works of early science fiction, Venus and Mars are inhabited.
This history of the world of "The Sky People" begins to diverge from ours around 1960 when probes to Mars and Venus reveal first that these planets are habitable, then that they are teeming with terrestial forms of life from all eras of our planet's history, and finally that these include Homo Sapiens Neanderthalensis and Homo Sapiens Sapiens. And everything else from dinosaurs to sabre toothed cats. Someone is using these planets as giant zoos for terrestial life forms.
Instead of concentrating on the arms race, the Soviet bloc and the USA compete to colonise Mars and Venus. At the start of the story, both the USA, with some support from the UK, and the Communist powers (including China) have bases on Venus. A soviet shuttle crashes, and although it contains a number of things which the Russians would rather the US did not know about, they are so desperate to recover the crew that they ask the Americans for help. Meanwhile the French have plans of their own.
There are a number of amusing references to events and fiction in our history: one of these is that Stirling has named one of the major characters in this book "Wing Commander Christopher Blair." (For those who never played space sim games, Christopher Blair was the main character in four of the five "Wing Commander" computer games.) However, Blair in this book is definately not the clean cut boyish hero played by Mark Hamill in the Wing Commander games. Indeed he's more like Admiral Tolwyn in a number of respects, starting with the fact that he appears to be a stiff upper lip British type who is not an entirely sympathetic character.
Overall this is a very exciting alternative history/sci fi crossover, which is as entertaining as Stirling's "Nantucket" trilogy and his book "The Peshawar Lancers". So far there is one sequel, "In the Courts of the Crimson Kings," in which the action moves to Mars.