Michael Gilbert 
Death In Captivity [PDF ebook] 

A suspected informer is found dead in a collapsed section of an escape tunnel being dug in a prisoner-of-war camp in Italy. So as to protect the tunnel the prisoners decide to move the body to another that has already been abandoned. But then the fascist captors declare the death to be murder and determine to investigate and execute the officer they suspect was responsible. It therefore becomes a race against time to find the true culprit and Captain Henry "Cuckoo" Goyles, a former headmaster, master tunneller and sometime amateur detective takes on the case.