Demon (pdf, epub) by Joel Abernathy

Falling in love with an obsessive serial killer was the last thing on my to-do list.
I was born into a world of villains, so it came as no great surprise when my brothers fell for the most twisted monsters of all. A sociopathic fixer. A psycho sadistic cop whose name belongs in the dictionary next to corruption.
Maybe I’m the one who’s crazy, thinking I could have any semblance of a normal life. The worst part is, I can’t even blame them for my not-so-perfect marriage crashing and burning, or my struggles as a single father. When my young son goes missing and a ransom note turns up demanding a trade for the serial killer my brothers-in-law keep locked in a basement, though?
Yeah, I’m blaming them for that. And I’ll tear the world apart to get my kid back–even if it means keeping Demon leashed at my side.
James is something far more dangerous than Silas and Malcolm. He’s an unhinged, self-righteous killer who’s on a mission to punish the sinners who broke him in the first place.
He’s also my only hope of ever seeing my son again. When you embark on a journey through hell, you want a demon on your team.
It’s when he ends up in my bed that it’s a whole different problem.