She Once Disappeared

Artemis Blythe is on the run with only a convicted serial killer as an ally.
In order to rescue the man she loves, chessmaster and FBI agent Artemis has to perform a task for the man she hates. The Ghost-killer's demand is simple: find Helen.
Artemis' sister has been missing for nearly twenty years, long thought dead. And now, in order to appease her father and his accomplice, Artemis must discover what happened to Helen Blythe all those years ago.
Once upon a time, she'd thought the Ghost-killer had murdered his own daughter.
But new evidence is found, and Artemis is no longer so certain... The truth, it turns out, is far more bone-chilling.