You did it! We figured this one out! It is a truth universally acknowledged (by me for certain) that the Bitchery pretty much knows everything, and really, it's true. Scroll down to see the solution for this HaBO - and many thanks!
This HaBO is from Emma, who is looking for this historical romance. Spoilers below:
First time HaBO request here. Looking for a historical, I think I read it within the last two years and I’m pretty sure it was a new release and the first in a series.
The heroine was a duke’s daughter investigating her friends suicide (she thought there was foul play and I THINK she was getting letters that also suggested this?) Anyway, somehow she ended up getting help from this super wealthy Covent Garden gangster-type. I think he was very well known in society because he loaned money to the ton or owned a bunch of businesses or something to that effect. I think he had a facial scar too. It’s not Sarah Maclean, but similar hero to the one from Wicked and the Wallflower. Oh, and I think he had a sister he hid in plain sight because his enemies were a dangerous sort.
I remember one scene where the hero and heroine both get letters from each other to meet in some dark alley but it’s a setup and they’re attacked. I think he kills the attacker and freaks her out a bit.
Total spoiler: the friend was murdered by her own sister because she was pregnant and it would have ruined her husband’s political career.
How can I remember all of this but not the name of the book or author!?
Sound familiar to anyone?

It reminds me of The Bachelor Bargain by Maddison Michaels. I’m a bit fuzzy in my recollection of the book details but what I recall fits.
Very curious to know the title of this book as well. This is definitely something I’d be into!
This sounds similar to “Taming of the Beast” by Christi Caldwell. The heroine isn’t investigating a friend’s suicide, but the whereabouts of lost children, since her family is responsible for the disappearance of one of them. She does engage the help of a former prison warden with lots of gang connections, and he does have a sister he keeps hidden away because he has lots of enemies. He does also save her from an attack in an alley that is a setup. Hope this helps!!
I second The Bachelor Bargain. I just re-read the “figuring things out” part, and it matches the spoiler perfectly.
Following. Following. Following.
Reminds me a bit of Janna MacGregor’s The Bride Who Got Lucky, but I don’t think Nicholas had a scar.
disregard my comment, in Janna’s book, the friend’s husband was responsible.
It was The Bachelor Bargain! Yall are the best!!!