River City Girls/Downtown

The effect in parenthesis only applies the first time you consume the item.

Book Bistro
The books are only effective in your inventory. To get the +2 WP bonus you have to "use" them, which removes them from your inventory, so the weapon durability disappears.

Merlin's Mystery
Its location is not marked on the map, but it's in the Tunnel to Ocean Heights. Press the light attack button in front of a boarded-up wall with papers on it.