Did you use beta readers? This is just one of our posts.
We talk more about the mechanics of useful books here.¹
Unfortunately, if it's not organically growing, it's not getting word of mouth, and if it's not getting word of mouth, it's just not that useful.
Either people don't have the problem you think they have, your book doesn't solve it, or the audience isn't right (in which case people don't run into people they can recommend the book to.)
Notes
¹ https://writeusefulbooks.com/resources/designing-nonfiction-...