Definitely look into skill trees and the Skill Links, some are really handy to have!
When you buy an Art Upgrade Book, go to the inventory and "read" the books.
Regarding the point of no return, Dunban will clearly say to you "this is the point of no return" and will ask if you want to continue or wait a little longer, I advise to wait and save, then when you do continue create a new save file for your game.