Here are two books I highly recommend. They represent the two major casting styles we use. These are both higher learning tools and a must for any serious two hand devote. The first is as free flowing as can be. Simple and self explanatory and not overly written. The second is a bit more technical in nature. You will want to have the Spey Dictionary at the ready and be willing to perform your best Thai Chi dance moves.
International Federation Of Fly Fishers Certified Casting Instructor CCI-THCI
Merrimack Valley Trout Unlimited
FlySpoke.com
Founder North East Spey Casters