I wrote a book[1] a year and a half ago that just recently crossed $42k in total revenue. These days it consistently earns $1.5k/mo without much further input from me, other than tweaks to the landing page copy and updates once in awhile when Stripe or Rails changes significantly.
[1]: https://www.masteringmodernpayments.com
Edit: If you'd like to read a preview, you can do so here: https://www.masteringmodernpayments.com/read