The key as you say with your police books is niche. I have published 51 books (2 with publishers) ALL the niche books made money(6 made over 100k) because I had large e-mail lists in those inches. I sold 99% direct not through Amazon. Therefore I have hardly any reviews.

ALL the non niche books did not make money.

One sold just 1 copy.

Then I partnered with another author who had a niche, renamed the book and boom instant success (six figures)

Still I struggle to sell my non niche fiction. But I keep trying :)