What to do? This forum thread offers helpful, hopeful hint.
"You may want to contact the taxi cab company. Esay to find if you remember what it was: G7, taxis bleus… Otherwise, if you have a receipt from the driver, its plate number should help the police to find the company name….You should go to the Lost Property Office in Paris:
Préfecture de Police,
service des objets trouvés,
36 rue des Morillons,
75732 Paris Cedex 15.
Any other lost notebook stories out there? And happy endings?