3 for $20 - The Couple Next Door by Shari Lapena

Your neighbour told you that she didn't want your six-month-old daughter at the dinner party. Nothing personal, she just couldn't stand her crying. Your husband said it would be fine. After all, you only live next door. You'll have the baby monitor and you'll take it in turns to go back every half hour. Your daughter was sleeping when you checked on her last. But now, as you race up the stairs in your deathly quiet house, your worst fears are realized. She's gone. You've never had to call the police before. But now they're in your home, and who knows what they'll find there.