I don't run my same books monthly. I run new releases. When I do re-run a book, I like to wait 6months to a year (BookBub rules follow this only allowing the same submission every 6months). Marketing monthly with newsletters is not very effective. When you think about it, the vast majority of newsletter readers, even with BookBub, are the same readers. They don't cycle with new readers for quite a while. But, since you mentioned it, Written Word Media will run your same book after 30 days. So it can be done. And I've seen the same cover over and over again from some writers in the same promo newsletters. So it is done. But, I suspect, not very effectively.
Ads are different. Ads run with new readers and casual onlookers popping up randomly all the time. So, the best method to achieve what you're looking for, imo, is ads: Bookbub, Facebook, Amazon, or Google ads. I do this with Bookbub ads.
For ideas with running promos more frequently, you might check some of these promo "hacks" published in an article by Written Word Media in 2022: 5 Ways to Hack Written Word Media Promos - Written Word Media
Ads are different. Ads run with new readers and casual onlookers popping up randomly all the time. So, the best method to achieve what you're looking for, imo, is ads: Bookbub, Facebook, Amazon, or Google ads. I do this with Bookbub ads.
For ideas with running promos more frequently, you might check some of these promo "hacks" published in an article by Written Word Media in 2022: 5 Ways to Hack Written Word Media Promos - Written Word Media
