The marshmallow test is an indicator of executive function (EF), This book is therefore an exploration of executive function. There are several interesting attributes that just a surface understanding of the results behind the marshmallow test wouldn't get you.
First of all, it turns out that willpower or executive function is contextual. Any examination of Bill Clinton's career, for instance, shows that he had plenty of willpower when it came to his most important goals, but was of course, undone by an affair with an intern. This pattern is repeated in many Hollywood movie stars, and even military generals like David Petraeus. What happened in those cases is that for such people, their focused willpower doesn't extend to certain areas of weaknesses which was what brought them down.
Secondly, willpower is not a fixed resource. It can be drained through use, so your willpower is strongest in the morning but weakest in the afternoon and in the evening. That's why bars, etc open late at night so they get more revenue when your willpower is the lowest.
Finally, executive function can be trained. There are several strategies that many adults try, including meditation, "mindfulness", and other well-documented techniques. Mischel goes through most of them and details the reasons they succeed (or fail), and how to make the more common ones more effective.
More importantly for parents, there are definite ways of bringing up children so that they have more or less willpower. In particular, modeling is critical, as is being supportive and not over-controlling:
The message here is that parents who overcontrol their toddlers risk undermining the development of their children’s self-control skills, while those who support and encourage autonomy in problem-solving efforts are likely to maximize their children’s chances of coming home from preschool eager to tell them how they got their two marshmallows. (Page 60)Not surprisingly, having absent parents or growing up in an unpredictable environment also leads to lower willpower:
Beginning in early childhood, far too many people live in untrustworthy, unreliable worlds in which promises for delayed larger rewards are made but never kept. Given this history, it makes little sense to wait rather than grab whatever is at hand. When preschoolers have an experience with a promise maker who fails to keep his promise, not surprisingly they are much less likely to be willing to wait for two marshmallows than to take one now. These commonsense expectations have long been confirmed in experiments demonstrating that when people don’t expect delayed rewards to be delivered, they behave rationally and won’t choose to wait for them. (Page 72)That means that keeping your promises to your kids is very important. There's also pre-natal advice, and advice that reducing quarrels between parents during the first few years is critical to having a kid with more executive function.
Overall, the book is good reading, though it's rather verbose in places and doesn't always do a great job getting to the point quickly. Nevertheless, it's got plenty of practical advise, and provides details behind the workings of executive function that a surface understand of what's happening during the test wouldn't give you, so I recommend reading the book for yourself.