Fire in the Blood by P.N. Elrod

Jack and Charles have gained some small fame for their skill in tricky investigations, so it's no surprise when they are hired by a wealthy man, Sebastian Pierce, to try to find out what has become of an heirloom  ruby and diamond bracelet stolen by one of his daughter, Marian's fast crowd of friends. Marian is a little faster than her daddy expects, and Jack finds this out on the first night of their hunt when she makes a daring play for him, just before her very jealous boyfriend walks in on them. With his vampire strength, Jack has no problem dealing with a jealous lover, especially paired with his nosferatu persuasive skills.

One little passage that readers who are not of a certain age will not understand:
"I hit the period and debated whether to turn it into an exclamation point...I backspaced, tapped the apostrophe key, and rolled out the sheet, adding it to the stack of deathless prose next to my portable."
You had to have been there to get it.

But the thief, McAlister, turns out to be more than just a thief. He and his accomplice have set up a basic blackmail racket - he seduces women of means, while his partner takes pictures, which are then used to extort money from the women. When he leads Jack and Charles on a hot chase through town, then turns up messily dead, the trail rapidly leads away from the usual suspects of family and close friends, deeper into Chicago's underworld - the mundane sort.

Escott and Fleming have some messy misadventures as they confront a seemingly endless host of gangsters who either want the pictures or the stolen bracelet, which is worth about $15,000 (in the 1930s). I think this novel led Elrod down some alleys, herself, and ended up spanning two volumes, rather than fitting into the usual page count - the story in the next volume continues moments after this one ends. Things get darker before the dawn, here.