Children's book on biological parents etc.

Hi Everyone.
My partner and I believe its time to tell our Daughter that her Dad wasn't her birth parent.
We have decided to try make her understand that some kids are adopted. Others have one biological and one step parent etc. Before we tell her that she is one of those kids.
I have herd there are children's books that help them understand.
Does anyone know the names of any books like that. I have been to the Library and Book Store but with out a Title they aren't much help.
Hoping you guys are a bit more helpful.
Wish me luck, have been dreading this day since she was born. I'm confident though with the right attitude and preparation on our behalf she will take it ok. Fingers crossed!

Thank You :)