We have a great book called "Gravity is a Mystery" by Franklyn M. Branley. It's a Stage 2 book as part of the "Let's Read and Find Out Science" series and it is awesome! We got ours at the local science museum, but I'm sure you could find it on Amazon. It has great visuals and great, friendly text that all my kids (5.5 through 11) really appreciate. I very highly recommend it.

She thought she could, so she did.