I found this on wikipedia:
The Amber Room by Steve Berry.
The story is about judge Rachel Cutler and her husband Paul, a divorced American couple caught up in a treasure hunt for the long-missing Amber Room. A couple competitive professional treasure hunters complicate matters. In their search through Germany to uncover the secrets behind its disappearance, they escape near-death in the tunnels running through the Harz Mountains, find themselves hanging off the edge of a tall church steeple, and discover a surprise in a hidden chamber of a Bohemian castle in the Czech Republic.
I almost bought this a couple of months ago but stopped myself as I had a big pile of books on the bedside table already - thanks for the memory jolt, will have to have a look for it tomorrow when I am in town

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