This is the first time I'm reading Boyle. I agree that it is a social commentary although a very sad one. It's useful to know about the history, culture, language, etc but Boyle's book tells me about the heart. The grass always seems greener on the other side - the tragic part is it never is. The wall metaphor is prevalent in the novel, starting from the title to the front gate and to all the closed doors in the estate.
Walls delineate boundaries even more clearly than lines.
But much more than the physical wall is the mental wall. Every conflict in the book so far happens when the boundaries are crossed, starting from the accident, when Candido, by sheer coincidence, gets picked to build the fence in the Mossbachers home, etc. I think Boyle captures quite acutely the pain that both couples experience as they try to reconcile modern day living while pursuing the American dream, each in their own way. This is a multi-part message in MIME format. The people who created housing projects professed to be helping the poor while what they were really doing was ghettoizing the people who least fit into society.