Eye In The Ring by Robert J. Randisi
Miles Jacoby is torn between a career in the ring and his new ticket as a private investigator. When his sleuth mentor is murdered, it's bad enough that Miles's brother is charged. Worse, Miles finds himself in love with his brother's wife.
The six Miles Jacoby novels are back in print in matched editions with new Afterwords by the author.
Said Elmore Leonard: "If Eye in the Ring moved any faster you'd have to nail it down to read it."