The Wraiths of War: Obsidian Heart book 3

Alex Locke is desperately trying to hold onto the disparate threads of the complex web of time he has created. He travels to the First World War, living through the horrors of trench warfare in order to befriend a young soldier crucial to his story; then to the 1930s to uncover the secrets of a mysterious stage magician. He moves back and forth in time, always with the strange and terrifying Dark Man on his heels, gradually getting closer to uncovering the true nature of his destiny with the obsidian heart.