I know that the name of this book implies that it's only about tigers, but it is actually about many animals and about their vulnerability. It highlights some animals who are extinct, those who are endangered, the ones who used to be endangered and came back, and why each of them was/are in the state that they are.
Can We Save the Tiger by Martin Jenkins is a nice introduction, with specific examples, to animal numbers and how we affect them. It really does give the feeling of how complex each situation is and how hard it may be to bring numbers up for some animals. Sometimes humans have to go way out of their way to help save a species.