I don't mind your decision, really. I'll just say this: Exposure doesn't pay your rent or bills.
Also, as a model (be it adult or otherwise), your mug and likeness are what pay your bills, because taking pictures is what you do to that end. Being in the book and on the book are two different things, in this instance.
I'm not saying be greedy, I'm just saying know your worth and use it. Also, as someone who works with photographers (since I'm a DJ who does parties and weddings), I know you're valuing the partnership, and I get it. Just remember that when your image gets associated with the book and where that can lead, positive or negative.