I think probably the best place to start would be with two French authors who fought at Verdun and wrote about their experiences. They would be Louis Barthas and Jean Giono.

Barthas, who would go on to win the Nobel Prize for literature, kept a very full diary during the war which ran many volumes. This was eventually published as Poilu: The World War I Notebooks of Corporal Louis Barthas, Barrelmaker, 1914-1918. Barthas was also a socialist and pacifist and his writing about the war reflects his strongly held humanist beliefs.

Giono also fought at Verdun and many of the main battles of the Western Front, but explored his experiences through fiction, writing a novel called To the Slaughterhouse in 1931. Although the novel itself is fiction, like Remarque's All Quiet on the Western Front, it is deeply imbued with the author's experiences in the war and has some pretty horrible descriptions of what it was like to be in battles like Verdun.

"We are nine in a hole. Nothing will get us out of here. But we have eaten, we must relieve ourselves. The first of us to feel the urge climbs out. He has been there for two days now, ten feet away, killed, with his trousers down. We crap on paper and throw it up and out. When we have no more paper, we go in our haversacks. The Battle of Verdun continues. We go in our hands. Dysentery flows between our fingers. We crap blood. We go where we lie. We are devoured by flames of thirst. We drink our own urine. If we remain on this battlefield.....it is because they won't let us get away." To the Slaughterhouse - Jean Giono