The book below is available in the Law Library, but SOLO searches will reveal many more in this area of research are in the History Faculty Library and other parts of the Bodleian.
The constitution is at the apex of the French legal hierarchy: legislation must be in conformity with it, and the Conseil Constitutionnel polices this.
The current constitution of France dates from 1958, and underwent considerable modernisation in 2008.
Following the Conseil Constitutionnel Decision 71-44 DC, 16 July 1971, Liberté d'association, Rec. 29, attention must be given to the sources mentioned in the bloc de constitutionnalité (the preamble.) These are the 1789 Declaration of the Rights of Man, the rights and principles recognised in the preamble to the 1946 Constitution and the 2004 Charter for the Environment.
Just some of the titles available in the LawBod.
Remember also to check for articles in law journals via IFLP/ILP etc