A small literature has grown up around Clairvaux, both for its monastic and its carceral history.
The Letters of Saint Bernard of Clairvaux (various editions, including a Cistercian Publications translation). Read a handful before visiting and the abbey becomes a different place — these are the letters of a 12th-century power broker writing to popes, kings, and would-be Crusaders from the cell where he died.
Le Dernier Jour d'un Condamné by Victor Hugo (1829). Hugo's blistering early novel against the death penalty was written when Clairvaux Prison was still relatively new, and the case of Sébastien Bornet — executed at Clairvaux — was part of what moved him. Reading this in the village adds an uncomfortable layer to the prison-abbey visit.
Vie de Saint Bernard de Clairvaux by Pierre Aubé (Fayard, 2003). The standard modern biography in French. Aubé is good on the contradictions: Bernard the contemplative who launched a crusade; the saint who could be ferociously combative; the man who insisted on austerity while building the wealthiest monastic network in medieval Europe.