Looking down at your body, you see that you have turned into a kangaroo. You also see that your clothes below the waist have been ripped apart by your powerful legs and new tail, and your shoes are in pieces due to your humongous feet. You feel your furry face with your prehensile front paws and find you have a snout and large ears.
Examining your body again, you discover you have a pouch. You put the spellbook inside it and practice hopping; first on the spot, then clumsy little jumps, and soon you are bounding around the park in huge leaps.