The Monks of Montafon

In the fifteenth book in the Wallace Boys series, it It is now winter, and the Wallace Boys have been recalled to the tiny European kingdom of Ruritania because because the Archduke Prince Karl-Franz is missing. Their investigation leads to a lonely monastery high in the Montafon Range of mountains on the border with Austria. Here they discover that the monks hold a terrible secret dating back to World War Two - the three boys discover underground mine tunnels directly beneath the monastery where the Nazis built their V-2 rockets using slave labour!