Arc of a Scythe

Neal Shusterman's trilogy is set in a future where disease, ageing and death have been conquered and an artificial intelligence called the Thunderhead runs the world. To keep the population in check, scythes are licensed to end lives. Citra and Rowan are taken on as apprentices to a scythe and must learn a job neither of them wanted. The story ends with The Toll; Gleanings is a later collection of stories set in the same world, best read after the trilogy.