His Forbidden Princess by Vivian Wood

Forbidden fruit is always the sweetest.

I was born a scoundrel. Destined to a life of poverty, until the royal family saved me. I owe them my life, and I’ll do anything to uphold my duty.
Even when my duty is babysitting Annika, the king’s little sister.
Except it turns out that Nika isn’t the wayward wild child I thought she was. She’s whimsical, stubborn, funny, gorgeous… and completely off limits.

Not only is she my best friend’s kid sister, Nika is a full ten years younger than me. While I was going to university, she was hanging posters of Justin Bieber on her walls and doing geometry homework. Don’t even get me started on the fact that she is a princess and I’m just a commoner.
And when I thought things couldn’t get more awkward? She asks me to be her first. Now every time I think about Annika, I picture her moaning my name.
My duty, my honor, and my sense of what’s right tell me not to look Nika’s way again… But my heart refuses to resist her.

The only question is, once I give in and take what I need… will having Annika just one time be enough?