Gravity

For the lower half of the intended age range (5-8, according to the publisher's website), it's not bad. It might be too simplistic for an eight-year old, but they'd probably still enjoy the illustrations of the concepts. It's very short and concise, and it illustrates the principles of gravity rather well and in ways that kids could learn about it (along with having access to specific language that would be used in science discussions, improving their vocabulary).

There's also a bit at the back to explain more, which is probably very helpful for adults who may need a refresher as they go through this incredibly short book. (Seriously, there's like 1-4 words per page, if that.)