Les Cahiers du CEVIPOF has a piece on the extreme right in France by Pascal Perrineau, available here (pdf).
Interesting factoid: Only 34% of FN voters consider themselves to be on the extreme right. 16% say that they are on the left, 12% on the right, and 9% in the center (!), while 28% say "neither left nor right."
Also: in a 2010 poll, 27% of workers favored the PS, 19% the FN, and 17% the UMP, making the FN the second largest "workers' party" in France, behind the PS but well ahead of the Front de Gauche (not fully organized at the time, to be sure) and NPA combined..