Compared to Harry Turtledove's other alternate history novels, this proved to be a let-down.
While offering an interesting premise of a world in which a German Second Reich has defeated the USA in the 1950's by using nuclear weapons, the story fails to properly explain how this came about, instead relies on using the very limited perspective of the two central characters, one a teenage boy from a more advanced alternate timeline which could be ours in the near future, the other a teenage girl from the German-ruled world.
The whole story is set in the alternate San Francisco, and leans heavily on various sketchily-described encounters between the two teenagers themselves and the local German Military Police, and also the local Triad gang, with the backdrop for this being the artifact shop run by the boy's father as a cover for exporting foodstuffs to their home timeline, and their need to avoid discovery by the authorities.
None of the people appearing in this story have any real characterisation, and there is no attempt to develop the historical background or to explain the cross-time technology. Nor is there any realistic depiction of the rest of the world beyond San Francisco, with the exception of the Germans, who are depicted as pastiches of Nineteenth-century Prussians, complete with spiked helmets, who somehow have been able to develop atomic bombs. Apart from a passing reference to China in connection with the triads, other nations are completely omitted.
As the basis for a pilot for a tv series this might be viable , but as a novel for serious readers of the alternate-history sf genre it misses the mark. I would not recommend it for anyone over 14 years old.