Obsidian Mirror by Catherine Fisher
The obsidian mirror. Its power is great and terrible. Men have been lost in it, the dead brought back to life through it, and the future annihilated by it. Or this is what will happen unless the mirror is destroyed. Three people seek the mirror: the first has been sent from the future to shatter its power; the second will protect the mirror at all costs, obsessed with its power; and the third needs the mirror to find a murdered father and save his life. But only one can succeed.
The mirror can send you to the past, but it will not bring you back.
Jake goes to drastic measures to get kicked out of his boarding school and get the attention of his godfather. Jake wants to get to Wintercombe Abbey where his father disappeared. And Jake is convinced that his godfather Oberon Venn killed him. Oberon and his father had been experimenting with a mirror that can send people back in time. But there are several people who want to claim the mirror (or Chronoptika) for themselves.
This is a unique blend of fantasy, time travel science fiction, and dystopia. Full of mystery, adventure, and distinct characters – Obsidian Mirror is both exciting and dramatic. The suspense builds to a climactic ending, but leaves us with many loose ends. First in a trilogy, this was a dark and surprising tale from a talented writer.