And yet the free source of marketing (your twitter account) doesn't have a link to the book.
Neither does your HN bio.
Your website is broken (expired SSL certificate).
As far as I can tell you don't have a single page that tries to sell the book. A link to amazon is a bare minimum.
https://writeusefulbooks.com/ is an example of a master class of marketing a book. It describes what the book is about and why one should buy it. It established the author as successful writer and therefore authority on the subject.
He also wrote an article good enough to hit HN and be a driver to the website. That required 0 budget.
Maybe your failure at marketing are a result of you being bad at it, not a universal truth that you can only be successful with "huge budget".