100 Heartbeats refers to the subjects of the book (...and upcoming MSNBC Documentary of the same title) species that have fewer than 100 members living in the wild.
The following is from the Acknowledgement:
It is my hope that this book will serve as a catalyst, educating people about the state of our natural world and compelling them to help protect it for future generations. We have the chance to do it and we can succeed. Every heartbeat matters.
The only thing that can make this better is the 16 pages of color photos promised in the published version...although Corwin does an excellent job of drawing the picture. This is a documentary that I look forward to seeing.