It has been approximately 3 years  since I  published the” Japanese Sentence Patterns Training Book” series.
https://amzn.to/33xGp0A
I checked the number of copies sold to date today. It was well over 1,000 copies (About 1,400 copies at present). The numbers of monthly average copies sold are 29.6 e-books and 22.8 paperbacks respectively. Though this is not just one book, but the whole series, it means that 50 books are purchased every month on average.
The big advantage of selling them on Amazon is that I can get feedback and comments not only from Japan but from readers all over the world.
I would like to continue to increase what I can contribute to Japanese learners.