So there are a few books I can immediately recommend and I'll try and come back to this later on with some more:
Smith, Leonard V., Stephane Audoin-Rouzeau, and Annette Becker. France and the Great War, 1914-1918.
Audoin-Rouzeau, Stephane, and Annette Becker. 14-18, Understanding the Great War
When dealing with a topic like this it helps to add some context to the French political situation immediately before the outbreak of war. In short there are widespread political factions in France; Socialists, Trade Unionists, Communists, Dreyfusards, Militarists, Nationalists, Bourbons (still a few of them left), Bonapartists (still a few of them left too!), Catholics, Protestants, Revanchists etc. And the one defining trait to them all is that they all hate each other.
The defeat in the Franco-Prussian War the French political system essentially has a pronounced existential crisis and collapse about what the future for France should be and what it means to be French.
On the left this had seen attempts at wider socialist internationalism with like-minded peoples across Europe. Many of these groups had met in conferences and various 'Internationals' in the early years of the 20th century where plans had been made that, in the event of a capitalist or imperialist war, there would be some form of general strike across Europe to bring the conflict to an immediate end.
But obviously this doesn't happen in any of the combatant nations and the obvious question is therefore; why not? With the spectre of war looming large in July 1914 there had been hopes among some French socialists that it could be halted. This hope however was dealt dual blows:
The French socialist leader Jean Jaures was assassinated on 31 July 1914. He had been due to attend another International conference to try and prevent the war but a French nationalist took exception to this and shot him twice thus decaptiating the movement.
The 2nd issue facing the Left was, it turns out, there was an additional unifying factor in France. Whilst everyone hated each other they also hated Germany slightly more for putting them in this position in the first place.
When it became clear that war was going to be imposed upon France whether they wanted it or not, the French population and state joined together into what became known as the Union Sacree (sacred union). This was efectively a unifying national movement to park their myriad differences in order to defend France.
The desire for this was solidified by the war being viewed, as described by Audoin-Rouzeau et al, as 'an eschatological conflict' ie; one that was apocalyptic in nature. If France were to lose the war then France, the home of civilization, would be destroyed by German kultur; it would be armageddon for all that was good in the world.
So the French left effectively get on board with the war effort (as do the other factions and, in fact, similar movements take place in the other combatants) at the outbreak.
This union sacree would last until 1917 when the French army, weakend and effectively radicalised by the battles of Verdun and the Chemin des Dames, mutiny in a movemebnt to reassert their rights as French civilians against what they see as an oppressive state that does not value their lifes or their political identity.
Whilst the French army would remake itself after 1917 and eventually win the war, the trauma of the experiences and the unsatisfying outcome of Versailles would greatly weaken the French political balance in the inter-war years.