Multiple Man
(need to look up the author). An interesting look at multiplicity and possession with a set of case studies and an overview of the last hundred or so years of related fields. The case studies can be a bit dry at times, but are worth reading.
Internal Family Systems Therapy by Richard C. Schwartz (1995, Guilford Press, NY, NY)
A book about pervasive pluralism/multiplicity in everyone which also carries a therapeutic technique and model to carry family systems therapy to the inner family. No mention of "integration" in the index, at all. This person believes everyone is multiple, so is not encouraging people to deny it, mask it, hide it, or "cure" it, but to work with it. Highly recommended, even though I haven't truly read it cover-to-cover yet. (-XES)
First Person Plural: My Life as a Multiple by Cameron West, Ph.D. (1999, Hyperion, NY, NY)
A story-format book about Cameron's discovery that he isn't the only person in his body. An interesting perspective on multiplicity, it has some of the same initial assumptions as mainstream psychology has (the trauma model, for example), but goes in very different directions from there on in. A worthwhile read.

See Also