Book 5 of the "Our Man Adelaide" Series:

It is 1937. Having seized Manchuria, Japan is invading the rest of China, and Adelaide DeMarcy, and her lover, William Wentworth, are doing what they can to aid the Imperial war effort. But General Bushida has another mission for them;
to travel to Nazi Germany, and there, help dissidents to smuggle influential Jews out of the country, and into Japanese-held territory where they will be resettled.

Yet one these Jews has disappeared. A brilliant metallurgist, he has spent years analyzing the samples from the Valley of Death, and they must find him before the SS can use his knowledge to create weapons of terror such as mankind has never imagined...