Honestly the best book/video/etc... is written by mother nature... Spend some time during the off season, deer season, etc.. observerving wild turkey behavior. I will say that a good video on calling techniques and sound files will help your calling, but ultimately, you will learn the most from the turkeys themselves. I am a full time turkey scholar, all year I spend thinking, watching, practicing, dreaming, and eating/sleeping turkey hunting and turkeys.

I also pittle with some custom calls, nothing fancy... just in my garage.
I think that helps me stay tuned into turkeys and their vocals!!!