Taking the book in hand, you flip through the pages until you find a page pertaining to Greater Monster Shift.
Judging by the list of examples, you think it would be a good idea to maybe try it in a more open space.
Thankfully there's a forest trail nearby that not a lot of people know about. Maybe you could try it out there.
It took 20 minutes to get to your destination, a large forest clearing you look around one more time to make sure you weren't followed. Once you were sure the coast was clear, you reopened the Book of Myth, read over the spell, and uttered the command, all with the image of your chosen form in your mind's eye.