Sarah Langan's fifth novel, “A Better World,” is set several decades in the future. In Plymouth Valley, the toxic air is continuously scrubbed clean by a filter called (perhaps a bit too ominously) “the Bell Jar.” An on-site farm grows the kind of fresh produce that has disappeared from grocery stores elsewhere. And while the unemployment rate outside the town’s high walls is 25 percent, everyone lives in material comfort and without the use or need for money.