Now I'm getting confused. I agree that it was an unknown quantity before I began reading it.
The main point I was attempting to make is that some of those (perhaps a very small minority) interested in learning Meteor operate on very tight budgets. On a good month I get a $50 "allowance." Most months I don't get an allowance. This month my wife agreed to take the cost of the Discover Meteor book out of our grocery budget. So when things are that tight, $30 or $40 is a lot to swing. It's roughly 10% of our grocery budget (for context).
For those who make their living as web developers and have sufficient disposable income, I'm sure the asking price of $39 isn't a problem.
I was confused by his phrasing. I thought he meant it was an unknown quantity because he had not read it.