I thought this may be of interest to HN since the author posts here occasionally and there has been past discussion of his book/typesetting framework [0].
I personally found "Butterick's Practical Typography" very compelling, and I happily paid for it (apparently well above the median selling price!). I'm a little disappointed to read that Mr. Butterick has only made something less than $15k over the course of the first year; while he seems cheerful about it in the post, I can't imagine that such a payout comes close to covering his opportunity cost for creating a high quality book.
Economic structures that support high-quality open-access content seem to be an ongoing challenge with no clear answers.