Tome of Training that is used by Blacksmiths to train and increase the mastery of their craft. These pages are needed in order to level up Your Blacksmithing. So if you are wondering why you haven’t found any in Nightmare, no worries.
These pages only drops in Hell mode and beyond.
When you upgrade your blacksmith it’ll come to a point where he requires 5 pages and gold to upgrade more. You need to have the number he requires in your bags. They are also required for some craftables. You need them in your inventory when you tell him to craft an item that need them.
Nightmare = Page of Blacksmithing
Hell = Tome of Blacksmithing
Infero = something else