I have to take a break from reading these. I prefer if there's story with romance rather than story in service of romance. I just felt somewhat unsatisfied by the end of the story and wanted more.
As usual, this is my opinion, your mileage may vary, everyone experiences stories differently.
Will Thorne's sanity is being drained away by the artificial heart in his chest. He goes to Holly Evernight, the person who put it there, who he blames for his difficulties. Now many people want her dead and she's hiding in her house with her experiments.
To me it felt like the author wrote herself into a corner and then had to use a lot of handwavium and deus (or faerie)ex macina to solve the problems. I was just left wanting more.
I would be stretching things a little to call it horror; it's supernatural, romantic suspense, darkest London; ghost; werewolves; monsters; terrifying women.