The Rhinemann Exchange by Robert Ludlum

David Spaulding is the most feared and efficient Allied agent in wartime Europe. Expert, deadly and professional, he is also high on the Gestapo's 'most wanted' list. Now Spaulding has been selected by the Allied Command to transact an undercover deal in Argentina involving top secret Nazi scientific plans. The dealer is Erich Rhinemann, an exiled German Jew who is awaiting the end of the war with his millions in an impenetrable retreat near Buenos Aires. But there's something Spaulding doesn't know. The other side of the deal. And it involves the most bizarre, horrific intrigue of the Second World War ...