I am working on creating a Quest Book. I have all the core functionality down, but I can't find a good way to store the data. I've decided NBT is the best idea, but I have no clue what things I can attach NBT data to, and what would be the best thing to attach it to? The data can't really be added to the quest book itself because what if they loose it? Also I want to data to be the same for each player, so preferably something that the data  could be saved to on the Server Side? Can somebody suggest what would be the best thing to do?

I answered this in your previous thread.

Thank you, sorry I never saw the World Saved Data section! Thank you!

This article mentions manual synchronisation between server side World Save Data and client side, how would I manage this? Would I just regularly have to update them all?

Quest progress should probably be saved using a Capability attached to the player, not using World Saved Data.

 

Whenever a value changes that the client needs to know about, send a packet to update it. The site I linked previously also has a section on networking, including the Simple Network Implementation (which you should use).