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.)