The sequel to K Pax (http://www.goodreads.com/review/show/23324832). It concentrates more on the doctor's idea that prot is the alter-ego of Robert, and trying to uncover the traumas that made Robert retreat (partly with hypnosis), but otherwise it doesn't add much to the story; reads like padding for the middle section of the first book. The balance of the first book is gone; although prot's extraordinary powers are noted and unexplained, the presumption is very much that prot is an alter ego of Robert's MPD.
The trilogy concludes in K Pax III: http://www.goodreads.com/review/show/23324833