Maybe you haven't so many, still you have a few.
Besides the classical ones like "Her Private We" by Frederic Manning, "Under Fire" by Henri Barbusse, "Storm of Steel" by Ernest Jünger (this one is more a first personal account ) and "All Quiet in the Western Front" by Erich Maria Remarque, you've modern writers basing some of their fiction in WWI.
The Regeneration trilogy ("Regeneration", "The Eye in the Door" and "The Ghost Road") from Pat Barker is one fine and best examples. (As long has you don't mind a certain Homosexual content)
You've also "Bird Song" from Sebastian Faulks, "Flanders" by Patricia Anthony, "The Unknown Soldier" by Neil Hanson (this one isn't really a novel) and "A Long Long Way" by Sebastian Barry. These are the ones that I own, but I'm sure you'll find more.