Slowswift

All right, I'ma crowdsource some worldbuilding 'cause it's giving me too much of a headache.

Consider, if you will: In front of you is a book. This book is magical. Oh, on the outside, it's nothing special: a nondescript leatherbound volume, a little worn but in good condition. Maybe it's a famous wizard's journal, maybe it's a grimoire, maybe it's just an outdated almanac. But something about this book has the potential to end the world if it fell into the wrong hands. What is that something?