Bawdy and aging Broadway star Sally Ross and her tough secretary, Belle, who cling together against the realities of lost loves and life passing, enter a world of bizarre terror when a persistent fan's letters turn perverse, obscene, and menacing
Reviews with the most likes.
https://www.theguardian.com/books/2017/jun/14/top-10-modern-epistolary-novels
I read this when I was 18 - 19 not long after release and can hardly recall it but do remember liking it. The link posted reminded me of it.
