I was pretty much wrong on all counts.
This book focused on the marriages more than the First Ladies. It was sort of glimpse inside the lives of twelve couples who happened to live inside a "fish bowl" called the Presidency. The book details the behind-the-scenes dynamics of these couples starting with their early life together, their climb up the political ladder, and how they managed as a couple within the confines of the Presidency.
Achieving the job of President requires a certain talent, ego, and luck. Historically it seems that it also takes a special pair of people to manage the supreme effort it takes to get that job and to survive in it. It was abundantly clear that no matter how dysfunctional the marriage was, the strengths of the first spouses balanced the weakness and tempered the responses of the person holding office ... sort of like two sides of the same coin. It was an interesting study in human nature.
It is worth a read if you are interested in a behind-the-scenes look at marriages under unusual circumstances.