Internal Family Systems
The Internal Family Systems Model (IFS) is a form of individual therapy focused on thinking about the mind as being made up of many parts, each of which can be examined individually, and how those parts work together. It was created by Dr. Richard Schwartz.
It can be very useful for plural systems, since it looks at everyone as being made up of a set of interrelating parts.
See more about it on Wikipedia, the IFS Institute, and Psychology Today.
Books
No Bad Parts - Healing Trauma and Restoring Wholeness with the Internal Family Systems Model
A useful guide to Internal Family Systems therapy, walking the reader through exercises to get to know their inner selves. While not just helpful to plural systems or those who think they may be plural, many of the techniques and concepts that the book explains can be used effectively between headmates.
Find out more at the IFS Institute here.