The burning girl / Mark Billingham.

Enhanced descriptions from Syndetics:

Some fires never go out ...

X marks the spot -- and when that spot is a corpse's naked back and the X is carved in blood, Detective Inspector Tom Thorne is in no doubt that the dead man is the latest victim of a particularly vicious contract killer. It's morbid and messy -- but it's a mystery with plenty of clues. This is turf warfare between North London gangs. Organized crime boss Billy Ryan is moving into someone else's territory, and that someone is ready to stand up for what he believes is his.

Thorne's got plenty on his plate when he agrees to help out ex-DCI Carol Chamberlain rake through the ashes of an old case that has come back to haunt her. Schoolgirl Jessica Clarke was lit on fire twenty years ago. Now, Gordon Rooker, the man Chamberlain put away for the crime, is up for parole, and it seems there's a copycat on the prowl.

Or perhaps it's someone trying to right a serious wrong: Jessica Clarke was the victim of mistaken identity. The intended target was the daughter of a gangland boss, a woman who would grow up to marry the current leader, Billy Ryan ...

Thorne quickly identifies a tenuous link between the two crimes, and past and present fuse together to form a new, horrifying riddle. One that involves more killings, violence, greed, and a murderous family with no values -- except gain at any price.

When an X is carved into his front door, Tom Thorne realizes that fires, once thought to be out, continue to burn.