Cantrips (and now looking like all spells) in the Spellbook tab cannot be added normally
Cantrips can only be added to a Spellbook by dragging it to an open, castable spell slot and not in the stored spell book area of the spell tab.
Cantrips can only be added to a Spellbook by dragging it to an open, castable spell slot and not in the stored spell book area of the spell tab.