From Kelly Burke:

You can use this web app to write and sell books on your own domain. We use it for our own book, which teaches you how to build the app from scratch: https://builderbook.org/book

Pretty cool. Writing your documentation or book in Mardown is amazing for a lot of writers. The reading experience on the web could be a bit better, but the app comes with all the third-party integrations you’d want to sell your book.