Over 80% of the world population lives on less than $10/day. The percentage of the world population who can afford this book is very small.
Maybe 10%, 5%? Other than repeating this obviously wrong statement, would you like to provide some evidence that >20% of the world population can afford this book?
The worlds population living on $10/day, will have no use for a book on a niche aspect of type systems. The majority of people on that wage will probably not even access to a computer.
Most people, at any income level, would not find value in this book. The fraction of people who find value in the book maybe correlated with income.
There are a large number of people who might benefit from access to this book (or similar books) would can not afford to legitimately purchase it.
Given that only a small fraction of the world population can afford this book (I estimated 5%, happy entertain other estimates) it seems likely that the majority of people that could benefit from access to this book, could not afford to buy it.
Maybe 10%, 5%? Other than repeating this obviously wrong statement, would you like to provide some evidence that >20% of the world population can afford this book?