Cuckoo Song, by Frances Hardinge.

The premise: Triss wakes up not feeling like herself. Her parents assure her that she is just ill, and had fallen in the “grimmer”, but will be all right. Yet the little girl cannot remember important details of her life, and is ravenously hungry all the time, causing her to eat and eat and eat to exhaustion, while never gaining any weight. And her sister claims she isn’t Triss, but a monster tricking them all…

Fascinating book. Gripped me from the start, and sets a claustrophic, paranoid mood. Really great stuff so far.