Taking the book, you flipped through the pages, until you come across the spell for Monster Shifting.
Like the summons, they also have levels of power, from Lesser to Legendary and Divine.
Still a bit unsure about the magic this book holds, you think it wouldn't hurt to humor it by reciting a spell. After all, if nothing happens, you can assume this was all an elaborate prank.
With the tome in hand you thought of the creature you wish to become, and uttered the command to Shift...