I would be okay with that as a monetization model, except that the book author despite being a self-described FOSS dev doesn't seem to have anything to do with the project (https://github.com/beancount/beancount/graphs/contributors).
Ah, not quite true. The author fixed a typo in a docstring once (https://github.com/beancount/beancount/commit/8584763b618f76...).