Elizabeth Greenhalgh is an eminent diplomatic-military historian of Allied relationships on the Western Front in WW1, as shown in her previous works on Franco-British relations (Victory through Coalition), as well as her work on Foch, who became the generalissimo of the Allie armies in 1918 and throughout the war was extensively engaged in cooperation with the British, Belgians, and Italians. This forms the context for her work devoted explicitly to the French Army, The French Army and the First World War which aims to explore both the general development of the French army but also to focus on its context in the war, diplomatically and in the mic coalition war where relations with allies were vital.
Readers who have previously read Robert Doughty's Pyrrhic Victory: French Operations in the Great will find many parallels between the two works. Both aim to explore the broader context of French military involvement in the war, on peripheral fronts, and to demonstrate that there was a genuine French military strategy. Pyrrhic Victory however, has a more concentrated framework, which much emphatically pushes the notion of the French following a unified strategy and stubbornly sticking to it, that of a containment of Germany and crushing offensives on all fronts which the Central Powers would lack the resources to respond to.
The French Army and the First World War by contrast seems to show the French army as more reactive, trying to respond to different contexts and with different factions pushing for different policies, such as the Salonika Front, pursued as a result of the same split between Westerners and Easterners which was so noticed in Britain during the First World War. It does provide a better sense of the Battle of the Frontiers than other works and Joffre's intentions for a push at the
center of the German line, believing it to be much weaker than it actually was, while other books do not give the sense of the decisive moments and objectives of the battle.
A tremendous amount of detail has been assembled in the book, covering the various operations and battles on the Western Front in particular at great length. This includes some sections which are little covered elsewhere, such as the 1914/1915 battles in Alsace (Hartmannswillerkopf), fierce mountain fighting which saw dismal combat in freezing cold snow, between tenacious French and German troops mostly forgotten today, even compared to the gaping hole of the rest of the year, between the Battle of the Marne and the Race to the Sea and then the Battle of Verdun in 1916. It also gives details about French defensive operations during the Second Battle of the Marne which are not covered elsewhere, and which put into excellent relief the capacities of the French army on the defense in the last year of the war, when it operated according to the principles of defense in depth and not in its rigid defense of the Chemin des Dames, adopted due to a bad local leadership. Most of the book is rather dry matter-of-fact, but in sections like this when it includes quotations and stories, such as the German soldiers and their response to the battle and the terrible conditions of the offensive some humanity and a feeling for how the battle felt for the soldiers. Perhaps Greenhalgh meant to have more since she mentioned at the beginning that she meant to follow four different individuals in the army, a mixture of pilots, soldiers, and officers, but this rarely comes up throughout the rest of the book. Its ending covers some of the broader details of social transformations and the army and its evolution, but it really seems like it should have been more lengthy and detailed thing than the relatively short chapter that feels like an addendum tacked on at the end: with a title like the French Army and the First World War one expects a more broad history of the army than just its battles, and the ending only partially compensates for this There is also a lack of maps which would do a lot to help put its battles into better context.
There has already been a quite massive amount of detail assembled in Pyrrhic Victory, which Greenhalgh herself admits at the beginning of the book - in fact more, since Pyrrhic Victory, is a lengthier book, and it covers the material and arms production of the army in detail than Greenhalgh's book. The French Army in the Great War seems like a different version of Pyrrhic Victory, taking many of the same themes but coming later and without the extra general history which makes up Doughty's work, designed mostly for being part of a series of armies in the Great War which Cambridge published. This isn't to attack its own merit or accuracy: it covers combat operations very well, and discusses the relations between Western Front allies in detail fitting to Greenhalgh given her diplomatic subject, but if you have already read Pyrrhic Victory, there isn't much that will be surprising. For ultra specialization on the French army it makes sense to read but otherwise Pyrrhic Victory generally already covers everything the book.