So is the takeaway here that there is no need to put any skill points from leveling up into Arcane Lore unless one wants the 28 spellbook?
Also, what about Nature Lore?
Thanks, figured out which one I missed. It was the one from the inner treasury at goldcrag... tried to kill the sentinel but he was way too hard at that level. Thought I'd come back later but totally forgot about it.
There are a number of fights where you seem to be permanently or almost permanently immobilized in the later part of the game. Definitely annoying! Getting 2-3 summons plus 1-2 turrets and spacing them all out helps to keep the damage off your main characters though and buy you enough time to kill things. Teleport scarab and other teleportation abilities can help for positioning too.
In some cases pretty dramatic bonuses. I have Nathalie at 13 focus mastery and 15 ward mastery and she's pretty damn close to immune to elemental damage.