Yeah that joke ran its course the other two times someone used it."Now do Me" - Erza Scarlet, 2013,
Yeah but I meant his spells. Like, why would an ubber powerful, super dupper dark mage put all his knowledge in a book knowing it would eventually find its way out of his hands? And why hasn't anyone copied it just in case? The purpose behind it is weird for me. Though I'm happy that the origins of FT are being developed and worked uponnot his magic, I think rather his spells... like when the 2nd master created those demons.
If Tartaros has the Book of Zeref which creates demons and he is in Tartarus and he slays demons, the Devil Slayer may be a good guy or there is some crazy explanation that no one can predict for his status.