I'm sure these have come up before, but DD7 and I are reading "George's cosmic treasure hunt" by Stephen and Lucy Hawking and she is eating it up.
It got her thinking so much that I now have to find an astronomy professor at the local university who is willing to talk to her about some of the ideas brought up in it because I do not know the answers to her questions, and I know that some of the questions don't have answers yet, but she's not willing to believe me that even the astronomers don't know some of the answers yet!
We already read George's secret key to the universe and she loved that one too.