Also if you were to somehow poll everyone reading this and ask "Did you buy this book?" you'd get some number, x. But if the book were priced at $5, then $5y would be much greater than $40x.
I bought a bamboo fineline pencil for $50 the other day. It's a tool that will serve me for at least a year. It's unclear whether this book would.
I want work like this to exist, and for the author to be rewarded for it. But ultimately, in an era when words are infinitely and instantaneously copyable, the economic value of words seems to drop.
Given the choice between stealing knowledge and not stealing knowledge, when you wouldn't have paid for it anyway, where's the harm?