The Knife of Never Letting Go by Patrick Ness
In this story by Patrick Ness, we follow young Todd Hewitt as he and his dog Manchee flee through swamps and forests, over hills and through plains, searching for an unknown place of safety, with nothing to his name but a small bag and a knife. Stumbling upon Viola, a lost girl from The Old World, the two are on the run from Todd's old settlement, and more specifically from its large army that is out to capture him.