This book, Eternity in Their Hearts is a missionary classic. It is filled with missiology (the study of missions) and  it is filled with examples of people who were waiting. Waiting for hundreds of years trusting in prophetic promises given by their own ancient prophets. This book is thrilling to say the least. You will have a hard time putting it down once you start. If you have an interest in missions, you must read this book. If you have a burden to pray for missions, you must read this book. You will sit in awe at God's soveriegn work among the unreached tribes of the world