For any students interested in French linguistics or sociolinguistics, here are the books that I recommend for an introduction as well as a more in-depth explanation. You don’t necessarily need to have a background in linguistics to be able to understand everything, especially for the first three books.
Exploring the French Language by R. Anthony Lodge, Nigel Armstrong, Yvette M. L. Ellis and Jane F. Shelton
French: A Linguistic Introduction by Zsuzsanna Fagyal, Douglas Kibbee, and Frederic Jenkins
The French Language Today: A Linguistic Introduction by Adrian Battye, Marie-Anne Hintze, and Paul Rowlett
A Sociolinguistic History of Parisian French by R. Anthony Lodge
French: From Dialect to Standard by R. Anthony Lodge
Unfortunately a few of these books are a bit more expensive (mostly because they only exist in hardcover). Hopefully you can access them electronically through your library.
Social and Linguistic Change in European French by Nigel Armstrong and Tim Pooley
Social and Stylistic Variation in Spoken French by Nigel Armstrong
Sociolinguistic Variation in Contemporary French edited by Kate Beeching, Nigel Armstrong, and Françoise Gadet
Recommendations for books written in French to follow.