DD enjoyed The Crazy World of What If by Steve Parker--it's not at a very high level, but it asks some fun hypotheticals and explains the science behind the answers. Although I guess it is a little scattershot, but DD liked that. A higher level book that she also enjoyed is Science Explained by Colin Ronan (that I think I maybe heard about here--so thanks to whoever recommended it in the first place!). Both are available on Amazon.