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.
As you collect more and more cards, you will start to fill up Monster Card Sets within the Codex. Each set can give the Codex bonus stats that will apply to your character, but only the bonuses of one set at a time may be used. As you complete a set, it will give your Codex some experience, and with enough experience your Codex can level up with your character, providing higher stat bonuses.
The other main function of the Crusader Codex is to store your monster familiar cards. Mobs have a rare chance to drop familiar cards upon dying, and if you pick these up, you can use them to add them to your Codex. You can them summon that Familiar at any time to fight alongside you and give you buffs.