They were Winston Churchill's "secret army", tasked with causing havoc behind enemy lines had the Nazis invaded during World War II. But keeping the identities of the Auxiliary Unit members a secret was crucial to their survival. One of the ways was by hiding bomb-making manuals in plain sight, disguised as old calendars and diaries.

Its cover promises agricultural fertilisers that "do their stuff unseen until you see results".

But turn the page and a comprehensive guide on where to plant explosives and how to make your own bombs is revealed.

The Countryman's Diary 1939 is one of two manuals being auctioned later which shed light on the training and abilities of the guerrilla units....