Talk about mixed messages.
Tinuviel accepted the book on reflex before she fully understood what Maleficent was implying. Even when she'd gone over the words in her head, it made no sense - not in that she didn't understand, but in that she simply couldn't belief it. Obey her? Well, she couldn't deny having certain... ambitions regarding the Academy, but to put her charge so suddenly, even if it was only in charge of one mission (or whatever was written inside the book)... There was no way she could have predicted this.

"Mistress..." she spoke demurely, "what about the sacrifice?"