Well, I don’t know about you, but I thought Jack Miller and Sophia von Naundorf had made it through to peacetime at the end of Agent in Peril.

Not a bit of it – they still have their most exciting and dangerous mission before them, and what could be their most effective operation.

But, thinking about it you couldn’t really end a successful franchise like ‘Wolf Pack Spies’ after just two books. So you will be glad to hear that Jack and Sophia are back – though I have to say that I had become so fond of them in the previous two books, that I was a little scared for them both as I opened the latest Gerlis.

This time, much of the action takes place in Berne in Switzerland – where the agents and their British handlers are based – and in Lyons, where Klaus Barbie, the so-called ‘Butcher’ of that city has taken up residence.

The opportunity emerges for Sophia to impersonate a senior Gestapo representative on Barbie’s staff, so she really has to take it. Only this time, while she is searching the archives for clues about who betrayed Jean Moulin – de Gaulle’s representative, trying to unite a disparate resistance movement – in June 1943, they also find that their arrangements and cover-stories are less well-organised than before, because there is also a mole in among the British handlers in Berne.

This leads to the unexpected death of one of the best-loved British spy masters. I won’t give it away or be a spoiler by saying who it is.

There is something about the way that Alex Gerlis writes that makes these books so compelling. His prose is sparse – there are few, if any, judgements about the terrifying world he is describing – but you learn to like many of the characters.

And especially Jack – an American sports writer – and Sophia, the widow of a senior SS officer, who had both become British spies.

And the British are portrayed as you might expect – terribly, terribly public school and only completely at ease with each other when they went to the same schools, or their fathers knew each other. Yet you somehow love them for it anyway.

There are only a handful of historically real characters in the book, and only Barbie is actually at the heart of the action. He is much younger than anyone expects, as befits a man who lived to be convicted of crimes against humanity in Lyons, as late as 1987.

Yet the events that are described as the background to the book, and the techniques which were used by spies – and the dreadful decisions they had to take in order to survive in wartime – are all highly accurate and authentic.

Gerlis has done his research. Though he says at the end of the book that, even in fictional form, the issue of who is Source Armand, the identity of whoever betrayed Jean Moulin, is still so fraught in France that he was twice warned off writing about it.

For that fact alone, this book is worth adding to your collection.

David Boyle is the author of The Xanthe Schneider Enigma Files (of which The Berlin Affair is part) and Munich 1938.