Book 18 on my Kindle. SS-GB by Len Deighton.
I was always under the impression that I'd read this, but, on 're-reading' maybe I hadn't. Perhaps it was the other one. You know, that guy who wrote the other Nazi counterfactual one. That one.
I enjoyed this a lot. Atmospheric. Like the bit I highlighted:
"The silk curtains were closed now, as they had been then, and the fire was not much lower, and yet the interior had changed in the way that all rooms change at night."