This book is actually a composite of two different concepts, which the author doesn't link to each other in a clear manner.
The section "shamanism in modern medicine" is poorly written. Most psychologists (like the author) hold that everything encountered in altered states of mind actually happens in the mind. Some Jungian psychologists softened this approach by postulating the existence of archetypal symbols. Conversely, shamans across the world and across millennia hold that there are parallel universes that are inhabited by independent spirits who may, and do, interact with this world. Who should we believe to, the scholars or the ... people in the business? Regardless, Achterberg completely disregards the "other" point of view and, in so doing, she fails to deliver objective information.
On the other hand, the section about mental imagery and healing is truly excellent. Self healing is a phenomenon that cannot be possibly denied, the placebo effect being the prove of that. The author explores the possible connections between imagery, the nervous system and the immune system. Most of what she says makes perfect sense, although current scientific evidence is not sufficient to support her thesis. The author points out that there is a strong economic pressure against research in this field, and she is perfectly right. Anyone working for a pharmaceutical company (a tremendously powerful lobby) would never, ever look favorably to research aimed to prove that the mind CAN heal the body.