I have a lot of bullet point summaries of nonfiction that I share on https://littlerbooks.com. I got my first hundred users mostly by just mentioning in it in social media posts where appropriate.
What we are currently doing: 1) Cold outreach to power users - to convert them into affiliates. 2) Cold outreach to individuals who have target ICP communities. 3) SEO for more long term (not for the first 500)