Perhaps 2 stars is a bit harsh, but I can't bring myself to give it 3.
It's not that it's a bad book; it's that you're dropped into what, essentially, is a conversation between Einstein and a young girl. There's no plot, no character development.
Some of the ideas posed are very interesting and provide good jumping off points for thinking about science in general, and of time, space, physics in particular. But lacking a story to ground the conversation, I didn't grow attached to the girl and found myself distracted, knowing that there wouldn't be any real resolution and that the conversation would simply end.