No I haven't read the book.
It seems cells were initially taken as part of a biopsy during treatment (I think it's fairly certain she consented to the treatment). But the cells that have survived were taken with a new technique after her death.
I wonder what the form would have had to say about allowing her cells to be used and whether she would have agreed to it or if it would be understandable? I suspect the vast majority of people even now would just tick that box which is why I think this is all noise.
From a legal perspective, the cells belong to whoever paid for the treatment and did all the work and training and outlay for equipment, staff, pioneered the technique etc...
Her decedents? I doubt they did much to really contribute to it all but they should of course be reimbursed if they do take part in any research.