The four books above were written by major French authors who actually fought in WW1. Another one I like particularly is La Vie des Martyrs, by another great French writer, Georges Duhamel. Duhamel was a field surgeon in WW1, notably at Verdun (if you can imagine what this means...). This book is really heart touching, really impressive. It has been translated to English as "The New Book of Martyrs":
https://www.amazon.com/New-Book-Martyrs-Georges-Duhamel/dp/1612031838?SubscriptionId=AKIAI6MZ6I56PEDTRFEQ&tag=worldcat-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=1612031838