I would suggest beginning with Chapter XX in Swords Around a Throne: Law and Order: La Force Publique and L'Armee de l'Inerieur.

Anything on the Gendarmerie and Douaniers would be very helpful.

Further, these might be of help:

-Memoires du General Radet edited by A Combier.

-La Gendarmerie Francais en Espagne et en Portugal by Emmanuel Martin.

-Gendarmes and the State in Nineteenth Century Europe by Clive Emsley. Parts I and II are the most useful along with Chapters 8 and 9 of Part III.