From the introduction:
Imagine time as a river covered in ice, a rampaging current pushing everything from past to future, with no way out. Now pretend that something had poked through the icy cover - something strong enough to crack the ice of time. In certain times and certain places, there are holes in the ice - small islands where things don't work like they should, where the world itself seems not-quite-normal.
For centuries they've been considered mere curiosities and nuisances - places where magic is dead, and even the power of the gods won't tread. Whole maths have been created by wizards to predict where these islands will arrive next, so they won't be taken unawares. Scholars have debated their origins, and their meanings. The gods themselves aren't talking - out of fear, or maybe just embarrassment. But that's all changed.