Symbolism, Imagery, Allegory
The Grimmerie is a book of power, destruction, and mystery. We're never really sure exactly what it is or where it came from. Does its existence mean that magic exists in our own "Other World" as well as in Oz? Did Oz make the Grimmerie magical? Is the book inherently dangerous, or are the people using it the ones who are dangerous?
The Grimmerie is symbolically linked with the Wizard, and thus with themes of power and corruption. It's rather ironic that Elphaba uses the Wizard's book to conduct her experiments in genetic engineering. Ultimately, she uses the Grimmerie to play god, much as the Wizard wishes to. In fact, it's never fully explained why Elphaba, respecter of animal and Animal rights and dignity, conducts experiments on monkeys to make them fly. Her "speech therapy" sessions with Chistery aimed to prove he had a "spirit," but the flight experiments seem to have no such motive. However, the fact that Elphaba uses the Grimmerie for these experiments highlights its dangerous power and potential corruption.