The Marrow Requiem

In the world of Kaldrun, entire cities ride atop colossal, half-alive Titans that migrate endlessly across a dying wasteland, kept walking only by the songs of bonewrights who can hear their pain. When apprentice bonewright Sera Thale discovers her vanished brother's heartbeat still echoes deep inside the Titan's marrow, she must choose between silencing the one song holding her city together or saving the only family she has left.