The target audience of this book is: 1/rich people; 2/people who would like to become rich; 3/people who enjoy dealing with ideas.
There are of course degrees of being rich. This book is for all degrees of rich people, from the super rich on down. It is for billionaires, for millionaires, but not just for billionaires and millionaires. There is no income qualification for reading this book. You may, for example, be earning only $100,000 or even only $60,000 (or less) a year, and yet you may well be rich when your income is compared to the population at large, or to your co-workers, or to your usual associates.
Other people in the target audience may not be rich yet, but you're planning to be rich. This book is definitely for you too. You need to be thinking about the ideas in this book--whether you successfully go on to become rich, or whether you just go on to earn a decent living.
The third category of people who are in the target audience for this book are people who like to read, like to think about ideas, like to search for truth. You may have a relatively small income, and no particular plans ever to become rich, but you are definitely part of the target audience as well.
Rich people, don't feel cheated that this book is for more than just the already rich. You can afford to share with the not-yet-rich, and with the tattered handful of people who simply enjoy dealing with ideas.