At the beginning a tapestry of laws of localities, from mid (15th there are steps being made towards a "modern" national legal system.
A north-south divide (La Rochelle-Geneva) can be traced in early French law. In the south (sometimes called "le pays de droit ecrit") the influence of Roman law remained strong; to the north the influence of the Germanic system was stronger.
In 1454, ordinance of Charles VII decreed that a written record should be made of all existing oral local custom. (Coutumiers term for collections of Coutumes locales.)
Under Francois I (and Chancellier Poye) the Ordonnance de Villers-Cotterêts (1539) introduced reforms across a wide range of legal matters, including criminal procedure.
Gallica - the online catalogue for the digitised collection of the Bibliotheque Nationale de France - is an excellent place to find early editions of the various coutumiers available on the free web.
The Advanced Search Screen allows you to do a subject search
Some subject searches to try in SOLO:
Customary law - France
Law France History
Law France Sources
Feudal Law -France
Land tenure -- Law and legislation -- France -- Early works to 1800
Feudalism - France
Take note of the location in your search results, you will probably find that most are in other parts of the Bodleian, not the Law Library. However, the Law Bod does have some titles which may help, such as those below:
Custom, Law, and Monarchy by