The Judge's List by John Grisham

 Judge's is apparently a sequel to The Whistler, which I must have read but don't remember. But it's a good story on its own. Lacy works for the Florida Board of Judicial  Conduct, which typically investigates judges with drug problems or mental health issues. But one day she hears from a woman who is convinced a sitting judge has committed not one but multiple murders over the past 20+ years. And her father was one of his first victims. Lacy is reluctant to get involved but the woman, Jeri, is very persistent.

She is also kind of an annoying character. Lacy is finally following the clues to try to gather enough evidence to take the case to the FBI when Jeri foolishly starts sending anonymous notes to the Judge, which clues him to the fact that someone knows what he's been up to. And since he's smart enough to have avoided detection for over 2 decades, he's smart enough to narrow down who it could be and figure out that Jeri, the daughter of an early victim, is the likely culprit--which puts her and Lacy in jeopardy. You'd think after she finally got someone to take her seriously, she wouldn't risk the whole case--and the life of the person who was helping her--for personal gratification, but you'd be wrong. Plus she never tells Lacy that she has been sending the Judge anonymous notes, which I thought was kind of cowardly. As you can tell, I liked the Lacy character better than the Jeri character. I wonder if there will be a sequel to the sequel.