The Emperor's Candlesticks - Baroness Orczy

When a group of Russian anarchists kidnap a Russian prince in Vienna there are repercussions. On learning that the Cardinal d'Orsay has agreed to convey some hollow candlesticks from the Emperor to the Princess Marionoff in St Petersburg, two spies both see the possibility of using them to convey messages safely into Russia. One is an eager young idealist involved in the plot against the prince, the other is Madame Demidoff, a beautiful agent of the Tsar. When the candlesticks go missing at the border, the two engage in a race to get them back, both realizing that their very lives could depend on the retrieval. (Summary by Wikipedia)