An excellent book on the origins and first months of W.W. I came out in 2014 called “Catastrophe” by Sir Max Hastings.
I would also highly recommend a book called “1913”, about the state of the world in all of its major capitals just before the war. The Great War was an unimaginable catastrophe that should never have happened, and destroyed what should have been major advances in peace, prosperity and freedom around the world.