Suggest me an alt-history sci-fi book with a bit of mystery sprinkled in (if you can).

I'm honestly looking for a bit of a unicorn, but I'm excited to see what people come up with here.

I'm really into sci-fi, but my favorite sci-fi's include an alt-history vibe to it. Fallout is a great example. Sci-fi with a 1950's/60's vibe.

I'm super into the roaring 20's, gangsters, the kitschy stuff. Love a good mystery, crime noir 1920's pulp story. If I can find a blend of sci-fi with 1920's/30's that would be amazing. I think the word is "Decopunk" I saw get thrown around once.

Bonus points if the protagonist is female, and if it's in first person. Don't care about romance or spice or anything like that but I don't mind it.

And hey, if there's nothing like that, that's fine. I'll settle for a good, modern 1920's-core mystery!

Thanks!