The great cat massacre by Robert Darnton (chapter 4) refers to a policeman called Joseph d’Hémery who investigated not only 18th century philosophical books and the booksellers who sold them, but also the philosophers themselves. Gallica does not have his files digitized (yet?) but d’Hémery’s 14 letters to Nicolas René Berryer de Ravenoville and Chrétien Guillaume de Lamoignon de Malesherbes dated 1753-1764 are included in Electronic Enlightenment. If your library does not subscribe to EE, ask your librarian. What a rich source of information for 18th century studies and a fascinating historical figure.