I came across this fabulous manual for every therapist supporting those with DID or DDNOS:
Title: Coping with Trauma-Related Dissociation
(Skills Training for Patients and Therapists)
Authors: Suzette Boon, Kathy Steele, Onno van der Hart
Publisher: W.W. Norton & Company
Year: 2011
(Note: There may be a more recent edition.)