Melissa Stewart is the award-winning author of more than 180 science and nature books for children. She offers a wide range of programs for schools, libraries, nature centers, and conferences. www.melissa-stewart.com"
You can learn lots of things in simple little kids books. Here is basic comparative anatomy and physiology between people and fish. People can extract up to 25% of the oxygen in air, whilst fish can extract up to 80% of the oxygen in water (although it doesn't say how much that is). Some good but not amazing pictures of fish.
Excellent description of both the human and aquatic respiratory system as well as many other pieces of information about how fish function in water. The author describes how fins, scales, swim bladders and other parts of a fishes’ anatomy help it to swim, conserve energy and breathe underwater.