[1] The title is derived from an incident related in the book, when Marks was asked why agents in occupied Europe should have their cryptographic material printed on silk (which was in very short supply).

[2] Unlike paper, given away by rustling, silk is not detected by a casual or typical body search if concealed in the lining of clothing.

A major theme is Marks's inability to convince his superiors in the Special Operations Executive (SOE) that apparent mistakes made in radio transmissions from agents working with or in an alike role as the Dutch resistance were their prearranged duress codes, which it transpired they were as he alleged, and which fact haunted him.

SOE management, unwilling to face the possibility that their Dutch network was compromised, insisted that the errors were attributable to poor operation by the recently trained Morse code operators and continued to parachute in new agents to sites prearranged with the compromised network, leading to their immediate capture and later execution by the order of the command of Nazi Germany.

As a boy, Leo had begun his code-breaking with that used by his father, in noting the prices paid for second-hand books.