When you mention grief and loss many people immediately think of the five stages. This was Elisabeth Kubler-Ross' claim to fame and also her curse. There was so much misunderstanding about these five stage that she spent much of her time re-explaining her original premise.
On Grief and Grieving was written during the time that Elisabeth was anticipating her own death and in fact she died before the book was published. David Kessler, who had worked with her on a previous book, had the privilege to again co-author a book with Elisabeth. He was present at Elisabeth's death and was profoundly affected by her death and her life.
The five stages of grief: denial, anger, bargaining, depression, and acceptance are not a linear progression as explained by Kubler-Ross and Kessler. The stages merely are a means to help explain the process of grief but not meant to be used as a blueprint for everyone. The authors pointed out that grief is individual and messy. People have to be supported where they are in their grieving process not by some arbitrary time-table.