My vocabulary may be off. So, at the start of a game, you can travel to the green healthstone. you can then read one of the spellbooks and you will have an option of 4 spells to learn, and you choose one. What determines what spells are available? (why are they different game to game?). Also, are there "trees" of spells, so unlocking spells made by a certain material will lead to higher spells of that kind?
If there is documentation on this, point me that way and I'll read.