There are so many, but a few of my favorites are (and I don't have them with me, so this is from memory).

Frank Putnam (about 1986) Multiple Personality Disorder

Both volumes on the subject of dissociation in the Psychiatric Annals of North America

Colin Ross 2nd edition Dissociative Identity Disorder (partial title).

But the very best thing to do to study this subject is the training programs given out of the International Society for the Study of Trauma & Dissociation. The have in person trainings in some major cities, and a very popular online course with all the best articles.

If your interest is in EMDR and dissociation, there are a few of us giving trainings, which you can find at www.emdria.org.

There are also several books on ego state therapy, notably Watkins & Watkins, Phillips and Fredericks, and Emmerson.

I hope to soon finish a book on ego state therapy that has over 100 original cartoons in it to communicate key concepts.
I can see my memory is sketchy, so if you have trouble making sense of the above, let me know. I'm out of town until Monday giving a workshop in Kansas City, and leave for the airport shortly, so can't do the research right now.