While crohn's is not an autoimmune disease the book is still quite valuable as it focuses on how to develop a nutrient rich diet devoid of foods that irritate the gut and which nourish a healthy gut microbiota. This along with other lifestyle changes (stress, sleep and excersise) can have a huge effect on affect the immune system and how disease can express itself.
So, to be clear, the diet is primarily about healing the gut by removing irritants and increasing nutrition, and is a valid resource for anyone with digestive disturbances of almost any type.
The diet is a very well though out elimination diet, and the rational behind it is solid (as far as theory based upon research ever can be)
I managed to get it through my local library so it cost nothing, and found the explanations of how the immune system functions (and how dysfuncion is related to gut health) to be very useful in my healing and understanding of health in general and IBD in particular