Ziglar is one of the foremost sales/success writers and speakers ever.
I truly enjoy listening to him ... he learned it by literally doing it, selling some of the toughest-to-move products in some of the worst markets and under some of the worst conditions. And he did it honestly, with integrity and responsibility, and he is going to teach you how to do the same.
Definitely worth it. If you're in sales or sales management, you MUST lay your hands on every one of his books and tapes and just soak yourself in it, absorb all you can.