In “Just Between Us,” her second novel in as many years, Pittsburgh-based author Rebecca Drake seeks to re-establish the gripping, suspenseful ambiance of her previous book, “Only Ever You.” This time around, the action takes place in the affluent Pittsburgh suburb of Sewickley, where the relationship of four elementary school moms quickly shifts from latte-sipping gossip sessions to enmeshment in the horrible life of one of the friends.