As best as I can tell there is no talent bonus for getting witchcraft 5. I also noticed there aren't any skills for man at arms or expert marksmanship past lvl 15. I can say this because I have made it a point to check cyseal, silverglen, and the homestead everytime I reach a new level. I am currently level 17. Am I missing something obvious or there not end game skill books/talents in the previous areas?
