In the throne room, head to the back (behind the throne) and down to the master's chamber, where you can use the teleporter shrine to return outside on Sentinel Island.
You can jump up the rocks by the teleporter outside and run over where there are no overhanging branches in order to transform. You may also be able to find a spot on the path down, or jump off and transform before you get too low.
BTW, you may want to use the book yourself, rather than complete that quest.