I haven't even begun to discuss the great Cistercian writers of the past! Bernard of Clairvaux dominates medieval Cistercian literature similar to how Merton dominates the Cistercian landscape today, but Bernard is only one of many early Cistercians worth getting to know. Aelred of Rievaulx, William of Saint Thierry, Beatrice of Nazareth, and Isaac of Stella are among the writers of the past whose sermons, essays, and meditations are still worth reading today. If you want to get an overview of the Cistercian classics, start with Pauline Matarasso's anthology The Cistercian World: Monastic Writings of the Twelfth Century.
There's so much more to the spiritual life than reading your favorite authors, of course. And it's certainly an irony to consider the wealth of written resources available from a tradition so steeped in contemplative silence. But the writings of the Cistercians typically combine an authentically down-to-earth piety with a rich sense of lived spirituality. Such writings are a treasure to read.