In Chapter 10, Matt finally opens his cookbook and makes something. Why is he able to do this? What stage of grief could this represent?

There are five stages of grief, and Matt seems to have come to terms with his grief, so the next stage would be bargaining. Matt is able to open the cookbook because he has no alternative because the store is out of them. In opening the book, he connects with the memory of his mother and accesses things she left behind for him... something he would not have previously been able to do.