Pat Barker's Regeneration trilogy is fascinating, although a brutal look at WWI. The first book imagines the time where Wilfred Owens and Siegfried Sassoon were both committed during WWI for shell shock (Sassoon more for speaking out against the war).

I really like Allison Pataki's books.

I am a big fan of Sebastian Faulks. His books are often set during the WW2 period.

The Girl at the Lion D'or
Birdsong
Charlotte Grey (this one is great)

Love love love A Very Long Engagement, by Sebastian Japrosot. (Set in WWI France )