Night of The Fox: by Jack Higgins.

One of the few books (or for that matter films or television series) about the wwII as the channel islands experienced it.

British left the islands since after Dunkirk they could not protect them, either, and it was hard enough to protect the mainland what with the relentless bombing of London and the countryside, with Britain standing alone (Roosevelt had his excellent mind and heart in the right place and knew that to let Britain lose was end of civilisation this time, but to persuade various factions, either the isolationists with blinders to reality or fellow travelers of the nazis in US, was highly non-trivial) against the destruction of civilisation.

A few years later when US was in the war (and as the saying then went, the island of Britain was like an aircraft carrier that is so loaded with personnel and other equipment it is not sinking only due to the RAF planes taking off every minute - or something to that effect), and the preparations for D-Day were on, is when this story begins, with one British officer wounded and marooned on the channel islands, but with knowledge of D-Day so he cannot be left safely on island with any chance of being taken by the enemy in custody and tortured in prison.

However, they have a couple of aces up their sleeves, what with a British professor who is a good impersonator as a nazi and with help of a woman to give more credence to his presence he arrives to take charge of the islands, with more surprises waiting - as luck would have it there is another look-alike that arrives in place of a high level nazi, and the first impersonator naturally knows nothing of the second impersonator and assumes his secret is now known, so he goes to assassinate him only to find it is a Jewish man impersonating a high level nazi.

The escape having taken place, there is yet another extremely unpleasant surprise for the impersonators in store on arrival in British mainland, adding to the grief and emotional devastation the first impersonator has gone through before the beginning of the story, resulting in his distancing himself even more emotionally from all around. Working for his nation he cannot stop, but going almost dead within is the only way he can cope.