The Paris Vendetta by Steve Berry

From the publisher (source: The Internet Book List):
The only thing rarer than the vintage editions Cotton Malone sells in his Copenhagen bookshop is the time he actually gets to spend there. Retirement has been anything but relaxing for the onetime U.S. government operative, who's been drawn into one perilous adventure after another, crisscrossing the globe from the Sinai Desert to Antarctica, while racing to uncover some of the most precious secrets in recorded history.

Back home in Denmark, Malone's barely had a chance to rest and regroup after his last high-risk mission when trouble comes knocking again. Actually, it breaks and enters-in the form of an American Secret Service agent with a pair of would-be assassins on his heels. Malone has his doubts about the anxious young man, but narrowly surviving a ferocious firefight convinces Malone to follow his unexpected new ally into the night-and into another all-too-close encounter with certain danger.

Their first stop is the secluded country estate of Malone's good friend Henrik Thorvaldsen. The wily Danish tycoon's eyes and ears around the world have uncovered the insidious plans of the Paris Club, a cabal of multimillionaires out to manipulate the global economy. Only by matching wits with a murderous terrorist-for-hire, foiling a catastrophic attack, and plunging into a desperate hunt for the legendary lost treasure of Napoleon Bonaparte can Malone hope to avert international financial anarchy. But Thorvaldsen's objective is much more personal: to avenge at any cost the murder of his beloved son by the larcenous aristocrat at the heart of the conspiracy. Through the storied streets and cathedrals of Paris, a breathless game of duplicity and death will be played, all to claim a prize of untold value-or to suffer consequences of unthinkable magnitude.

This book is part of the "Cotton Malone" series.
Here are some other books from this series: