The Scarlet PimpernelRead about the other titles on his list.
In Baroness Orczy's play and novels, The League of the Scarlet Pimpernel is a secret society of English aristocrats, dedicated to rescuing French nobility from the guillotines of the French Revolution. The League is led by the mysterious Scarlet Pimpernel (aka arch-fop Sir Percy Blakeney) who signs his messages with a drawing of the little flower. The plant is itself a secret sign of bad weather, as its petals close up when atmospheric pressure drops.
The Scarlet Pimpernel is one of Peter Millar's six top spy books.