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

Magnus is a mage who always got yelled at by the Hero because he was deemed "useless." One day after getting fed up with his party he left, and upon leaving he met a merchant that sold him a mysterious book. It turned out that the book which was written in the god's language was actually an ultimate encyclopedia that had a lot of useful information on how to defeat the Demon Lord! Thanks to that book, he was able to know about boss monsters' weaknesses and raise his level more efficiently, data about important people, and it even had info on complete gourmet spots in any city! Moreover, by using his wits and scheming mind, he's even able to outclass the hero party! With the ultimate encyclopedia in his hands, he advances to the true route to defeat the Demon Lord! (Source: Mangasushi, edited)