The French Confection by Anthony Horowitz

The two protagonists are two brothers, Nick and Tim. Tim is the older. One day, they win a week-end for two in Paris. In the train from England to Paris, they meet two strange people. One is a steward, and the other one is named Erica Nice. When they arrive, they go and visit the Eiffel Tower. When they come back two people kidnap them. These people ask them what the steward gave them, but Tim and Nick don't understand. Finally, these two criminals drug Tim and Nick, and they leave the two brothers on the road. The police finds them and wants them to find these criminals before returning to England. The police tells them the code name for the boss of the two criminals is mad american. Tim and Nick find the mad american. It was the code name for “madameericanice.” The police comes and arrests the criminals, and Nick and Tim Diamond have solved another case.