A Fractured Mind: My Life with Multiple Personality Disorder (Robert Oxnam)

An autobiography of a person with DID. Oscar Isaac actually read this to prep for his role in Moon Knight, which got me to read this in the first place.
Each of the chapters are written by different alters of Robert's system. They tell the story of how he progressed from dealing with alcoholism and bulimia, to be able to connect with each of his alters personally.

Alters have enormous differences - in voices, demeanor, literacy levels, even heart rates. Most of all, they have their own identities and their own feelings. Deny that 'realness' and MPD therapy can't work.