I had a tiktok page and my book was due to people asking me to make a book based on my tiktoks. So I made the book and told them when it was done. I don't consider this advertising.
Congrats on your success by the way!
You make it seem as if you just posted your book on Amazon, did not mention it anywhere, and people started buying it.
You did market it via a direct channel to 300K people.