E. Phillips Oppenheim 
The Vanished Messenger [PDF ebook] 

A conference of European nations is being held in the Hague. England has not been invited to attend. Some think war is about to break out. Mr. John P. Dunster, an American, is traveling to the Hague with an important document that may prevent the outbreak of war when he mysteriously disappears after a train wreck in England. Richard Hamel is asked by the British government to attempt to solve the mystery of Dunster’s disappearance and prevent the outbreak of war in Europe.