The Crusader Codex is given to every character when they are first made. It is a one of a kind item that cannot be un-equipped, cannot be sold, cannot be dropped, cannot be given potential, and cannot be traded. While hunting monsters around Maple World, they will have a chance of dropping a Monster Card when defeated. If you pick up that monster card, it will be added to your Crusader Codex automatically and you can then go and look up information about that monster whenever you wish.