The Silent Girl by Tess Gerritsen

A VIOLENT DEATH: When a hand is found in an alley in Boston's Chinatown, it is detective Jane Rizzoli who finds its owner - a woman whose throat has been slashed so deeply that her head is nearly severed.

A REVEALING CLUE: Two strands of silver hair cling to her body. They are Rizzoli's only clues, but they're enough for her and pathologist Maura Isles to make a startling discovery. This violent end had a chilling prequel.

A SECRET BETTER LEFT BURIED? . . . Years earlier, a horrifying murder-suicide took place in a Chinatown restaurant, leaving five people dead. But one woman connected to that massacre is still alive: a mysterious, beautiful martial arts master who knows a deadly secret.

A secret that will kill again - unless Jane and Maura can track it down, and defeat it...

This was okay. I think I'm tainted by having watched the Rizzoli & Isles TV Series now tho ... Haha. Yeah, I didn't think it was an amazing twist. But this is probably fine for light holiday reading :)