The Slanted Worlds
In Fisher's whirlwind middle book in the Obsidian Mirror trilogy, Jake, Sarah, and Oberon Venn continue their fight for control of the Obsidian Mirror. After an explosion, most of them hunker down at the crumbling Wintercombe Abbey, desperately trying to master the Chronoptika. They are all there except for Sarah, who was sent from a ravaged future to destroy the device, and Jake, who is trapped in the London Blitz. They try to put aside their differences and agree to work together to save Jake and rescue his father. Whoever wins will either save a life, change the past, or rescue the future.