We pretty much just launched our startup (waitlist) and we published a full free ebook (about 200 pages) on a very specific topic for our audience: Revenue Operations (given our product is boosting sales productivity). This has helped us to kick start traffic to our website, while definitely not yet 20K, we see steady 4-5K daily traffic from it (no ads, no social).
When I was working on my previous start up I did the same. It's very successful but cost a lot of research and time to create. In general it also helps when you have a popover to 'download' the book (4.6% conversion rate for us) + you can break down the book into multiple articles (SEO).