Has anyone else noticed this in Night Witch? When Nightingale’s running over his escape plan? He’s got a book he’s reading in his cell, and he carries it with him as he disables the Russians, blows a hole in wall, collapses some stairs, and generally makes his escape. The book? The Metaphysical Poets.
Now, somehow I doubt his Russian captors picked out a book of poetry to keep him entertained during his captivity. So he probably had it on him when they picked him up. Which means Nightingale was just wandering around with a slim book of metaphysical poetry in his pocket.