This book by Harlynne Geisler , Storytelling Professionally: The Nuts and Bolts of a Working Performer is a must have for anyone thinking about going "pro" or for anyone who will hire storytelling professionals. Basic information, sound advice, and lots of examples make this a valuable resource. May need to look for it but it is worth the effort. Read a review: http://storyteller.swiftsite.com/stprore.htm.
Another classic in the field - it is more expensive and may also be hard to find. The World of Storytelling by Ann Pellowski. It is a more scholarly work covering the history, training, ethnic applications and cultural interpretations of storytelling around the globe and through time.