This is the prequel to Glass Castle which I have not read. I am planning on reading it now that I have read this one.
Half Broke Horses is about Walls' grandmother, Lilly. She was quite a woman. Her early years were spent on a pretty hard-scrabble farm in west Texas. Her family moved to a more fertile but still rustic farm in New Mexico when she was still fairly young. At the age of 15, she became a teacher in a one-room school house. The men were at war and the teachers and other women were working in the factories. Her stint as a teacher only lasted a couple of years but she was hooked.
This story takes us through Lilly's life until her daughter, Rosemary (Walls' mother) is married and has had children. Where Lilly was a grounded and hard-working woman, it's pretty clear early on that her daughter Rosemary is hell-bent to fight such convention every step of the way.
I understand that Glass Castle is a pretty dysfunctional story that can be a difficult read. I'm still looking forward to reading it now that I've started at the beginning.