IIRC while there are a small number of videos and audios of Milton Erickson, he never really made any training materials.
The best introductory hypnosis book I've read is "Training Trances" by John Overdorf and Julie Silverthorn. It is easy to understand and to the point. If you can find one at a reasonable price, get "Trance-Formations" by Richard Bandler and John Grinder. It is out of print due to some disagreement with the author and publisher, not to to lack of popularity. When it went out of print the price skyrocketed immediatly. "Patterns of the Hypnotic Techniques of Milton H Erickson, MD" Vol 1 and 2 by Bandler and Grinder are aslo good sources, but IMO are a bit dry and technical and are better suited to read after you learn the basics. "The User's Manual for the Brain" vol 1 and 2 by Michael Hall and Bob Bodenhammer, while textbooks on NLP, are also very good resources for hypnosis.
There are other good books but I'm only recommending ones I've read. I recently bought "Trancework" by Michael Yapko, and it looks like it will be a good book. It is a textbook on hypnosis at over 500 pages and written for therapists.
