Graham Day 
The Glowstone [EPUB ebook] 

Lux Fischer and Sparrow Highwater have a big problem. They’ve grown up reading adventure books. Now, they’re living in one.

One day, walking home from school, Lux finds a strange, glowing ring. The magic it contains doesn’t seem to follow any rules. And before she knows it, the ring has transported them on an adventure through time and space — threatening all of reality in the process.

That means Lux and Sparrow have to save not just our world, but all the worlds that have ever existed- but can they even manage to save themselves?