I recently bought the book and am finding it very useful. I don't believe I am DID, but I am definitely dissociative and know I have seperate ego-states ... and I do dialogue with myself out loud and in writing.
Gerlach's language of "subselves" makes a lot of sense to me and I can "give myself" permission to pursue understanding of my inner "parts" without having to use the DID label. Although his main focus is on working with divorced and otherwise fractured families, his basic theory about the wounded child and the inner "commmunity" are probably useful to anyone who experiences dissociation.