First Published in 1908, The Elusive Pimpernel by Baroness Orczy is the 4th book in the classic adventure series about the Scarlet Pimpernel. (Summary from Wikipedia)
Series:
1:
The Scarlet Pimpernel
2:
I Will Repay
2.5:
The League of the Scarlet Pimpernel
3:
The Elusive Pimpernel
4:
El Dorado: Further Adventures of the Scarlet Pimpernel
5:
Lord Tony's Wife
6:
The Triumph of the Scarlet Pimpernel
7: Sir Percy Hits Back
Prequel:
The Laughing Cavalier