The Found Child by Jo Crow
Years earlier, Elaine suffered the worst pain a mother could endure. Her infant son vanished. He's returned to her and his kidnappers are jailed. Seven years later, Elaine must face a new pain. Her son is diagnosed with leukemia. Worse, blood tests reveal he is not their son at all.
Despite her love for her son, she cannot help but wonder. If this isn't Jakob, whose child does she have? What happened to his parents, and why is her husband hiding stuff from her?
It's clear there is a bit of a complex mystery in The Found Child. I found myself torn, however. I wanted to buy that Elaine was a doting mom, but she did a lot of things that had me questioning her. She takes off while her son is undergoing cancer treatments. She's gone for hours at a time, if not a full day, and never really seems to be too torn apart that she's left him in the hospital with the husband she's suspicious of and a sister who seems to hate her.
I thought I had the story's outcome pegged. I was wrong. Jo Crow gets high marks for pulling a twist I didn't see coming. I love being surprised and this one did just that.