Towards the end of the Second World War, a Californian in Germany peered into a salt mine in Merkers, outside Frankfurt.
What Robert Thomas, of General Patton's Third Army, 90th Division, found in there greatly surprised him.
The Germans had used an area in the salt mine to store thousands of antique books. Thomas, then aged just 18, took two 16th century books as mementos.
This Tuesday, more than 60 years on, he will hand them over to the German Ambassador to the US, Klaus Scharioth.
Thomas said that the books were in similar condition to when found them, as he had kept them in the darkest and coolest place in his house.
The Germans had stored the books in the salt mines with a similar idea: because they were dark and dry. With the libraries at risk from Allied bombing, the underground mines were a far safer place.
Scharioth said that the return of the books was a piece of good luck, or "Gluecksfall", and that handing them over showed "a sign of friendship and trust."
One of the first editions was published in 1573 on the subject of Prussian statutes, and the other in 1593 as a commentary on Roman Law.
They are expected to be returned to the University of Bonn and the Diocesan Museum of Paderborn, to which they belonged before they were taken.