The book is about all losses. One is the necessary separation from the primary caregiver (mother) that is a natural part of becoming a separate person. Something we experience as a child and as a parent.
She also talks about connection, aloneness..all as part of the separate self.
It is a good book. Judith Viorst is a psychoanalyst and good writer, with a sense of humor, too.
A chapter about siblings entering the family is titled "When Are You Taking That New Kid Back to the Hospital?"