Whiskey Undone by Carrie Ann Ryan

Ainsley and Loch are best friends. He is a single Dad, gym owner, and security installer. She is a teacher, and she watches his daughter, Misty.
One of Loch's employees, Dennis, is found at the gym dead. Loch has an alibi from his family and Ainsley. He receives letters from an unknown person, claiming to the murder, and saying Dennis will not be Loch's only friend who is murdered. His former mentor, Jadon, died, and left Loch his entire company, to either run or disband. Loch has to keep himself, his family, Misty and Ainsley safe.
This is book 3 of the trilogy, can be a standalone, but I would recommend the first 2 as well, because they are both great books as well.
I received a copy in exchange for an honest review.