A detective fiction writer becomes convinced her lover’s death wasn’t a suicide and must prove his brilliant wife is a killer before becoming the next victim.

This is better because in the original logline I couldn't tell if it was a detective who wrote fiction novels or a fiction writer who wrote detective novels.

Added a new layer of paranoia to keep me on my toes.
“A paranoid noir author convinced her lover’s death was not a suicide sets out to find the killer before she becomes the next victim“

Take care.

There is definitely something to this idea but the logline is a little confusing. It took me a while to figure out just who everyone was. I think it needs to made more clear that her lover is a married man. I was wondering whether this story might be more interesting if the protagonist herself is married, and she's living with the person she believes might have killed her lover... but she can't let on otherwise she's admitting infidelity. I really like the two female leads battling it out so maybe she's a married lesbian and she has an affair with a man who ends up dead?

This has legs for sure! I think it's just about clarity at the moment. "Crime fiction writer" is much clearer, and I agree with sloanpeterson (SAVE FERRIS) that "brilliant" is possibly not the right choice but it's heading in the right direction. The idea of watching a thriller like this, mostly contained within one house, like a tense noir... yeah!! I do really want them to live with each other 24/7 though... but that might not be the story you want to tell.

For research check out Fincher's Gone Girl and Les Diaboliques. The latter is an amazing film which is PERFECT for what you're trying to write.

Hope this helps.

Funny you said that - I just tweaked to “crime fiction writer” and “detective wife” or maybe it should be “detective widow” but that sounds kind of funny to me.

I’m liking the idea of two strong female leads battling it out in a psychological thriller with one being an actual detective and the other being just a writer of these things.

This is confusing because it sounds like the protagonist has two professions, a detective and a fiction writer. It took me a while to figure out you probably mean a writer of fiction in the detective genre. There must be a less confusing way to write that.
I also think there might be a better descriptor for the wife than brilliant which to me sounds like talented.
Other than that, I like it.