TCI Online Store
Without a doubt, the best way to learn technical (rope and saddle) tree climbing is by taking training from a qualified instructor. You'll get lots of practice in the treetop, and your instructor will help you avoid the some of the mistakes and pitfalls of inexperience.
However, many people cannot afford the costs of tuition and travel for on-site learning. If this is true for you, here are some excellent alternatives and resources.