I’ve never thought of you as some client’s wife. Not for a minute.
-- Frank Lloyd Wright (Chapter 4 paragraph 9)

Importance: Frank is trying to tell Mamah that he loves her. To him she is more than a fling with a client's wife. What they have is substantial and not fleeting.

In a way , I’m relieved. We won’t have to carry on this charade anymore.
-- Mamah Borthwick Cheney (Chapter 6 paragraph 14)

Importance: Mamah is commenting on the fact that Catherine has found out about their affair. She is happy that they do not have to keep sneaking around. She believes it means that Frank will get a divorce and that she will ask Edwin for a divorce. They have talked about it many times. She is happy that there will be no more lies.

I need to get away too, Frank, from Edwin and Oak Park. And you. I need to sort things out. I have to find the path that...
-- Mamah Borthwick Cheney (Chapter 7 paragraph 7)