Discover Mammals is a short, educational introduction to mammals for kids.
This isn't a very in-depth look at mammals, but the book's targeted to younger children, so it works.
It covers the basics of what makes a mammal (hair, births live young, produces milk), goes over the difference between carnivores, herbivores, and omnivores, and gives a brief comparison between mammals and the other kingdoms. It even mentions the exception to the live young rule, in platypuses and echidnas.
...and my auto-correct is trying to change "echidnas" to "enchiladas". I'm pretty sure enchiladas aren't mammals.
Although I am pretty sure they're herbivores, since they're usually shown with vegetables nearby.
[A pair of enchiladas, seen in their natural habitat.]