The problem is many writers think of their newsletters only as self-promotion. And those newesletters come across as promotion (which is usually not enjoyable to read.) I have a free Substack newsletter. But only once per week. And I use it to send my old Medium fiction and humor stories to subscribers. They get new (to most of them) stories to read. And my old stories get new reads.

I occasionally promote my novel, but I always put it at the end where is can be easily ignored.