Wizard Squared: Book 3 of the Rogue Agent Novels

When the staff of Witches Incorporated receive a visitor from an alternate reality, they are shocked to learn that life in the parallel world next door is anything
but a bed of roses. And it's all because of Gerald Dunwoody.

At a crucial moment in time, their Gerald turned left - but the alternate reality
Gerald turned right. Now the parallel world next door is in the grip of terror,
staring down the barrel of a thaumaturgical war. A war that threatens to spill across
the dimensions and plunge every reality into nightmare.

The only person who can stop a rogue wizard gone mad is another rogue wizard.
But what do you do when another rogue wizard is nowhere to be found?