Depends on what you want. There are plenty of good books out there though. I read this one: https://www.z2systems.com/np/clients/nsa/product.jsp;jsessionid=D54AED0DD8DF114ACE520B16B8BDD3B8?product=4& Gives a lot of good background on the biomechanics and Bill Parry has his own theory, the Valsalva Hypothesis. He basically came up with it himself but it's pretty interesting to read about. I liked this book. https://www.z2systems.com/np/clients/nsa/product.jsp;jsessionid=D54AED0DD8DF114ACE520B16B8BDD3B8?product=3& This book by John Harrison is deeper and not technique based. It's pretty much all mental/emotional/psychological but I think really has a lot of poignant stuff in it. It's dense so try not to get overwhelmed. I've only read portions. But I think you can download a copy of this book online. Do a search for the title and try to find a PDF. I think minnesotda state or whatever college has that great stuttering reference site should have it.