All Quiet on the Western Front
Not my usual choice of book but I read it as part of the Penguin reading challenge. The writing style makes it a quick and easy read and as everyone knows it's about the horrors of war from a German soldier's perspective.
This was amazingly written, and incredibly poignant - an elegy of a bygone generation, to be sure. However, it was incredibly violent and plainly disturbing - not light reading. It's worth a go, and I doubt you'll be able to get through your GCSEs without reading it, but there are better WWI books to sink your teeth into - Storm of Steel by Ernst Junger is a good catch, as is Testament of Youth by Vera Brittain (also an excellent film).