This book is going to be incredibly useful for a certain type of person. That person is someone who meets these criteria.
1) Tends to see the potential pitfalls and dangers in situations
2) Uses this awareness to plan effective strategies to minimise those dangers in advance
3) Carries out these plans with low levels of fuss and distress and so manages to be productive and effective in what they do
4) Is sick of being told to 'stop worrying' or to 'look on the bright side' by those around them
For people in that situation this book is well researched, tightly argued and an effective tool.
The people who may find this book harmful are those or meet the following criteria.
1) Whenever something bad happens they think it will last forever, ruin al their life and be their fault or totally beyond their control
2) When thinking the way decribed in 1) become highly distressed or paralysed into inaction
For those people this book runs a serious danger of reinforcing their disabling thinking and may result in a significant danger in thinking style becoming seen as a virture.
Probably the best way to read this book is to read it alongside something like 'Learned Optimism' by Martin Seligman and to personalise the mixture of realistic optimism and constructive pessimism that suits you.
As you read this book please be careful. It does parody the 'positive psychology' movement as a Polly-Annaish way of thinking. True positive thinking doesn't say that everything will be wonderful, it simply says that even if the worst happens you have the skills and resources to limit its duration and extent of impact, and you can get many of the things that are important for you by applying strategies of realistic optimism.