The DS Ni no Kuni game, released exclusively to Japan, included a full printed version of the Wizard Companion book. Pre-ordering the PS3 Wizard Edition also rewarded players with a physical version of the Wizard Companion.
Marcassin is a long, dark-haired man with a light complexion, considered beautiful by many characters in the game. This could be a reference to "Howl's Moving Castle", an animated film also by Studio Ghibli, as Marcassin shares the appearance of Howl, a character with long, dark-hair, light complexion, and is also considered beautiful by many characters in the film.
In the ending of Ni no Kuni: Wrath of the White Witch, Spoiler:Oliver flies back into Motorville on a broom. This could be a reference to "Kiki's Delivery Service", an animated film also by Studio Ghibli, in which Kiki is always flying around on a broom.
There is a kingdom in Ni no Kuni: Wrath of the White Witch named "Nazcaa". This is named after the famous archeological site Nazca, in Peru.
One of the bosses in the game is a giant metal pig tank named, "Porco Grosso". This is a reference to "Porco Rosso", an animated film also by Studio Ghibli.