Plot-wise, most thriller writers spend their creative energies desperately trying to avoid painting their heroes into corners where they can't get out.

On the other hand, Bruce DeSilva, the Edgar-award winning author of a series about the wise, oft-hilarious Providence investigative reporter Liam Mulligan, just spent a year cleverly placing his protagonist into precisely one of those situations - and loved every minute.

In "Providence Rag," just out from Forge Books, Mulligan encounters and becomes part of an incredible ethical dilemma. Due to an antiquated state law, any juvenile convicted of a crime - regardless of the severity - must be released at the age of 21. Legislators never expected this to apply to someone like Kwame Diggs, convicted at 15 as the youngest serial killer in history. Now, though, Diggs is well past 21, and Mulligan and an idealistic young reporter uncover evidence that the penal system has been adding time to the killer's sentence for crimes he very likely didn't commit.

And why? Because police and FBI profilers know if Diggs gets out, he'll kill again. Mulligan and his colleagues, then, have to make a choice whether to stay true to professional ideals and risk freeing a sociopath.

"What I like about the story is that no matter which side of the issue you're on, you're condoning something reprehensible," says DiSilva, on the phone after finishing the West Coast leg of a signing tour. He'll sign copies of "Providence Rag" at a luncheon Sunday in Mystic's Bank Square Books.

Another interesting aspect of "Providence Rag" is that, unlike the first two books in the Mulligan series, it's based on a real case. In 1989, 15-year-old Craig Price, "the Warwick Slasher," was arrested for murdering four people in his neighborhood - including two children. By law, Price was scheduled for release when he turned 21, at which time he vowed to the Associated Press "to make history." But Price remains behind bars, having had his sentence continually extended by crimes he denies having committed in prison.

"The difference between the book and real life is that I don't know for sure if Price has been kept in jail because of funny business. In the book, they find out. I can only suspect in real life," says DeSilva, who was a journalist for 40 years before starting to write fiction. "I'm like Mulligan. He doesn't want Diggs out of jail, and I don't want Price out of jail." He laughs. "I particularly don't want Price out now. What if he doesn't like the book?"

But DeSilva turns somber when he talks about his decision to write "Providence Rag."

"The truth is, I debated doing it," he says. "I'd covered the Price case when I was a reporter and ended up writing a long magazine piece about it. I devoted a couple of months of my life to this dark psychology. I had young children at home, and it disturbed me to keep thinking about it."

It wasn't until DeSilva came up with the idea of focusing on the ethical aspect of Digg's incarceration that he decided to move ahead with the novel.

"There have been enough serial killer novels," DeSilva says. "But in the book, the crimes and apprehension of the killer all take place in the first 70 pages. The rest is the story of conscience and dilemma, and of course about the development of Liam Mulligan as a character."

Another important aspect to the book is that it takes place against the fading of daily papers and the watchdog mentality of print journalism - a subject very dear to the author.

"The crime novels I like most are the ones in which the writer uses the story to say something about society," DeSilva says. "But each of the (Mulligan) novels hopefully has something to say about culture and where we're going. Mulligan, of course, has a somewhat cynical view."

Is Liam Mulligan at all based on DeSilva? He laughs.

"All of my characters are completely fictional except, yes, there's a bit of me in Mulligan. It's just that he's three inches taller and 20 years younger."

Bruce DeSilva author luncheon, noon Sunday, Bank Square Books,

53 West Main St., Mystic; $30 for signed hardcover, catered lunch and discussion with author; reservations required: (860) 536-3795, banksquarebooks.com.