Locke and Key: In Pale Battalions #1

The impossible, reality-bending keys of Keyhouse have always been weapons of war. In the spring of 1915, Chamberlin Locke's oldest son, John, is desperate to be a part of the greatest war of all... and never mind that he's too young to enlist. He means to use the power of the keys to turn the tide, and will tell any lie, and try any manipulation, to have his way. Prepare to open a door onto one of the grimmest battlefields of the 20th century, whose darkness might even strike fear into an army of supernatural shadows.