The Other Mrs by Mary Robinette Kowal 

Sadie and Will Foust have only just moved their family from bustling Chicago to small-town Maine when their neighbour Morgan Baines is found dead in her home. The murder rocks their tiny coastal island, but no one is more shaken than Sadie.

But it’s not just Morgan’s death that has Sadie on edge. And as the eyes of suspicion turn toward the new family in town, Sadie is drawn deeper into the myst…. Read more.