Successfully complete the game once, then play again. New items will appear in various locations.
To get to the special stage, when you get Roan (King Roan) to be part of your team, exit the Palace. When you get out, there are two places to go to, Raul Hills and the King Roan's palace place in the south. Go to Raul Hills. When you arrive, "Special Stage" will appear. You can train yourself until you are ready to leave.
Book Of War:
To get the Book Of War, win the Walnut game six times. When you return from the game, they will give you the Book Of War.