It's a fantasy society without advanced technology, so complicated chemical treatments for the paper aren't possible. They've been making paper for a long, long time though, so they could have picked up other tricks to increase its longevity. The manuscript's caretakers use the best paper for the purpose that's available, and they store it carefully and don't handle it often.
Any ballpark figures?
Edit: Oops. The climate is coastal desert, so while it can be humid, it's not as humid as a coastal rainforest. The temperature where the book is stored doesn't vary that much since it's inside rather thick stone walls. No sun or elements. (See, they're careful.)
As for ink, they'd naturally choose the one that will last the longest.