Cuckoo Song by Frances Hardingle

This book was a finalist for the 2015 Cybils Book Award in middle grade speculative fiction. Triss doesn't remember anything about falling into a lake, but it's changed her. She sleepwalks and starts eating strange things, including her dolls. For some reason, Pen, her younger sister, hates her. Triss follows Pen to an old movie theater and discovers a man called the Architect kidnapped the real Triss at the lake. Pen made a deal with the man to get rid of her sister, but she now wants to end the deal. A fake Triss was created at the lake, and this Not-Triss has made things worse. Not-Triss will disappear in a few days, but it wants to help rescue the real Triss and find out what really happened to their brother in the war. first. Not-Triss and Pen form an unlikely team, as they travel to the Underbelly in search of the Shriker. This man created Not-Triss and may be the only person able to help the girls uncover the mysteries and stop the Architect.

The whole conflict with Triss/Not-Triss was very creative. I don't remember reading about a character like that. The relationship between Not-Triss and Pen was dynamic. I expected it to change but not to that degree. The book's conflict was one of those where I was wondering how the author could possibly get it resolved, especially with a time limit and the antagonist having all the magical power.  Understanding the Underbelly, its inhabitants, and the deal between the father and the Architect required abstract thinking. The book will probably be most appreciated by upper middle grade readers.