I haven't read the book, but my mother died quite suddenly when I was an adult.
My DS (16 months) was born 5 years after she died, and I do feel huge waves of sadness every now and then that she will never meet him, and vice versa.
I also feel a certain weight / loneliness at times from not having my mother around to lean on - the buck stops here. My friends, who have children of a similar age, are able to arrange to leave their DCs with grandparents and I have to admit to feeling more than a bit envious of that - not just because of the freedom it would give me just to be able to go out for a couple of hours (although that would be fantastic!), but also because their DCs have a family network that my DS will probably never have.
I think what I am realising now is that losing my mother meant the loss of much more than one person. I turns out that she was the glue that held a lot of my (mostly far-flung) family together. Since her death, I have seen my Aunt (her sister and only relative in this country) a handful of times, and the only communication I receive from my grandparents (her parents - they live a long way away, and have never met DS) is a card at Christmas. I write and send photos, but for all I know they never arrive. I don't think this is because they are deliberately avoiding me, rather that they are elderly and have their own problems and concerns.
I think when I feel sad at the loss of my mother, I am also mourning the loss of my family as it would have been. I have a fairly good idea of how different things would have been if she had been alive now, and it brings me to tears to think of all the additional people she would have brought into DS's life :(
I do sometimes think of myself as a 'motherless mother', but I don't think that defines me. I'm just me, I think, whatever that means. I'm not yet near the age that my mother was when she died, so haven't given that too much thought. Nor is DS near the age that I was when I lost her. I have wondered whether the fact that I have a DS rather than a DD makes a difference though - maybe I don't worry so much about history repeating itself because it feels different (if that makes sense).
I do have a DH, so I do have support and help though. His family live in this country, but far enough away that we are only able to see them a couple of times a year. He has had his own bereavements, and is a fatherless father, but from things he has said, I am not sure that he would ever think of himself that way.