Most of the books on French language can be found on the shelves under PC, according to the Library of Congress Classification System.
In particular:
PC2001-3761: French
PC2700-(3708): Dialects. Provincialisms
PC2813-(2898): Old French
PC3201-3366: Provencal (Old)
PC3371-3420.5: Modern patois of South France
PC3420.8-3495: Langue d'oc dialects
PC3721-3761: Slang. Argot