This post on HN generated about 2,700 views on the product page, and low double-digit sales. Sales have so far covered my direct expenses (domain registration,

This post on HN generated about 2,700 views on the product page, and low double-digit sales. Sales have so far covered my direct expenses (domain registration, cover design, copy editing), although I kept these costs as low as possible. Profit margin can only be determined by the value I put on my time. It will take a little while to cover this but the GTFS spec isn't going anywhere so I'm confident this book will be relevant for quite some time.