This was one of my favorites:
Another book that just occurred to me: The Great Escape by Paul Brickell. The true story of the British POWs incarcerated in Luft Stalag III, and later made into a blockbuster movie.
Possibly All Quiet on the Western Front. Or Five Chimneys by Olga Lengyel.
If War Lit includes actual histories, then Stalingrad by Anthony Bevor.
How about Maus?
I’ve thought that Fields of Fire is one of the finest war novels ever written. Further, the prose and vocabulary aren’t that complicated, it’s fairly straightforward. The author provides a glossary for some of the more esoteric Marine and Vietnam War era slang. The narrative is written in an episodic manner, with some flashbacks and flash forwards which might be tough to follow though. Plus, it’s fairly short.
I’m leaning towards All Quiet… for my student, because it’s quite short, it has a Sparknotes page (I have nothing against sparknotes, it’s all learning! As long as the reading still gets done.) and she can watch the film afterwards if she likes. I’ll see what she thinks.
I love all the other suggestions so much though, there’s a lot going on my Xmas list. I can’t believe I haven’t read Maus yet!
What exactly qualifies? Does it have to be first-hand accounts, or can it be fiction?
If historical fiction is in the mix, I might look at the Warlord Chronicles (story of King Arthur vs. Saxons), Saxon Tales(Saxons vs. Danes) or the Sharpe novels by Bernard Cornwell. Or possibly some of the Hornblower or Aubrey/Maturin novels.
For first-hand accounts, I liked “Forgotten Soldier” by Guy Sajer. It’s kind of like an “All Quiet on the Western Front” except in WWII on the Eastern Front.
Adding on, Immediate Action is Andy McNabb’s—the Bravo Two-Zero guy—memoir of his initial infantry service, selection to SAS, and service in Northern Ireland. I liked Bravo Two-Zero, and I thought Immediate Action was a nice bookend to the story. I have not yet read, but mean to, The One That Got Away, the account of the Bravo Two-Zero mission by another member of the unit and the only member of the unit to escape capture by the Iraqis, Chris Ryan.
Heading to Iraq, but a little later on, Generation Kill is excellent. So’re the episodes I’ve seen of the HBO T.V. series of it. Sebastian Junger’s War is a great documentary of a U.S. unit in the Konegal Valley, Afghanistan. The accompanying movie, Restrepo, is supposed to be quite good too. Staying in Afghanistan, the account of Navy SEAL Marcus Luttrell, Lone Survivor, is yet another glimpse of the Afghanistan War, along with some examination of a few moral dilemmas inherent in fighting a war. I found all of them easy to read and interesting.
Finally, Lothar-Guenther Buchheim’s novel, The Boat, and history, U-Boat War, are both very good at showing what it was like to serve (and mostly, die.) in the German U-Boat service during WW2. Plus, the movie of his novel isn’t too shabby either. See the Director’s Cut, if possible. Pack a lunch if you do.
Everything qualifies!
According to the teacher at school (I’m a tutor, so I don’t make the rules) it can be any war. Technically no children’s books or YA lit, but so much is cross-over that I don’t really take it as a rule. It can’t be allegory though, so fantasy WWII allegories are out 
For this thread, however, everything goes. If you think it’s war lit & you enjoyed it, share it! (And maybe mention it specifically if you think it would be good for a 17 y/o EFL student.)
If you feel like tossing poetry into the mix, these two have always fascinated me:
Dulce et Decorum Est (WWI) and
The Owen is in the syllabus, but the other one is a great addition. It’s short, so perfect to sneak into the lesson quickly, and as the syllabus is all WWI poetry it gives a really interesting contrast. You can see the intervening 30 years in the writing. That’ll be interesting to put to her. Thanks!
Anyone familiar with contemporary war poetry? How about the American Civil War (as I indicated early, it’s not something I am at all familiar with in that way.)
Roald Dahl wrote some good short stories based on his war experiences, collected in Over To You. You can find synopses here. They’re easy to read, with humour both light and dark; the general theme is of the psychological cost of war.
I almost forgot… “Gates of Fire” by Steven Pressfield is probably the most entertaining and interesting war novel I’ve read. Kind of poignant as well.
It’s the story of Thermopylae that “300” wishes it could have been, and the movie that I wish could have been made, instead of that cartoony garbage.
Another vote for The Red Badge of Courage.
If these are the qualifications, then I’d suggest The Snow Goose: A Story of Dunkirk, by Paul Gallico. It is more of a novella or extended story, but it has a nice plot that a young lady might enjoy. WWII plays a large role, especially towards the end; and Gallico, while not as gritty as Hemingway, might be able to serve as a stepping-stone towards that goal.
My dad gave me** this book** for Christmas 1965, and I would still enjoy reading it today:
It has everything from Gallipoli to Verdun to Lawrence of Arabia (and she could watch the movies Gallipoli, Paths of Glory, and Lawrence of Arabia afterwards). My personal favorite is the story of Felix von Luckner, aka the Seeadler.
I got this book at the same time as the one above, and it’s still another great read:
My favorite chapter is the one about Manfred von Richthofen, aka the Red Baron.
My suggestion is from the same author, different war:
I second this recommendation.
The Killer Angels by Michael Shaara is another civil war classic.
Another vote for the Red Badge of Courage.
At 4737 pages, it may be a tad longer than what the OP is looking for.