The Strongest Wizard Making Full Use of the Strategy Guide -No Taking Orders, I’ll Slay the Demon King My Own Way-

Magnus, a mage who always got yelled at "just shut up and follow my orders!" by the Hero because he was deemed "useless". One day after getting fed up with his party he left, upon leaving he met a merchant that sold him a mysterious book.
And it turned out that the book which was written in the god's language was actually an ultimate encyclop...