As promised, here's my unfamiliar words from The Pillars of the Earth, by Ken Follett. Doubt you'll be using any of these words in everyday conversations.

• undercroft: a subterranean room, especially a vaulted chamber under a church.
  "Then in spring, they would vault the undercroft, floor the hall above it, and put on the roof."


• verderer: an English judicial officer having charge of the king's forest.
  But toward the end of the afternoon, Tom saw smoke rising above the trees, and found the home of a solitary verderer, one of the king's forest police.


• transept: the transversal part of a cruciform church that crosses at right angles to the greatest length between the nave and the apse.
  The crosspiece consisted of two transepts which stuck out to the north and south either side of the altar.


• obedientiary: one of the minor officials in a medieval monastery appointed by the abbot.
  He was one of the obedientiaries, the senior officers of the monastery.


• quire: the part of a church appropriated to the singers.
  Nearest the crossing was the quire, with wooden stalls where the monks sat and stood during the services.


• almoner: a onetime official of a monastery charged with distributing alms.
  ... there were three more officials who were nominally under his control but had a degree of independence, the guest-master, the infirmarer, who looked after old and sick monks in a separate building; and the almoner.


• merels: an ancient game for two in which each player has from 3 to 12 counters placed at the angles of a figure consisting of three concentric squares and tries to be first to secure a row of 3 on any line.
  The game was obviously a variant of merels, or ninemen's morris, and probably a gift brought back from Normandy by Aliena's father.


• tetchily: irritably or peevishly sensitive.
  "I know what battlements are for," the earl interrupted tetchily.


• jongleur: an itinerant medieval minstrel reciting and singing for hire.
  "Yes, he was a jongleur. He told me all those poems, just the way I told them to you."


• voussoir: one of the tapering or wedge-shaped pieces forming an arch or vault.
  The apprentices, among whom was Tom's stepson, Jack, built the arch up from either side, with the wedge-shaped stones called voussoirs.


• aspergillum: a short-handled brush used for sprinkling holy water.
  Each bishop carried a small brush called an aspergillum and a vessel of holy water, and as they marched, singing, they dipped the brushes in the water and sprinkled the walls of the church.


• compline: the last liturgical prayer of the day said after nightfall or just before retiring.
  Philip put down the template. "I must go in to compline." He turned away.


• barbican: an outer defensive work of a city or castle; especially, a tower at a gate or bridge.
  It had a stout stone wall with a castellated parapet, and here, where previously the bridge had led straight into the main street, the way was barred by a stone-built barbican with enormously heavy iron-clad doors that now stood open but were undoubtedly shut tight at night.


• shriven: to have been freed from guilt, especially by confessing one's sins to a priest.
  He had watched her weaken, he had seen her eyes close, he had heard her breathing stop, and he had let her die unshriven.


• justiciar: a high royal judicial officer in medieval England; especially a justice of one of the superior courts.
  While the earls and bishops and abbots met in the keep, the lesser nobility gathered in the castle courtyard: the knights and sheriffs, minor barons, justiciars and castellans; people who could not stay away from the capital city while their future and the future of the kingdom were being decided.


• posset: a hot drink consisting essentially of sweetened and spiced milk curdled with ale or wine, and sometimes thickened with bread.
  William, who normally ate and drank heartily, was nibbling bread, and sipping posset, a drink made with milk, beer, eggs and nutmeg, to calm his bilious stomach.

Koontz novels are always good for a nice list of unfamiliar words, many of them sinister or dark in nature. This book is about a nice man, a nice woman, and a nice dog, and a bad, bad man who wants to kill them.

• deliquesced: to melt away: disappear as though by melting. Buildings blurred into one another, traffic flowed sluggishly, and streets deliquesced into gray mists.
  


• prie-dieu: a small kneeling bench designed for use by a person at prayer. In scores of towns, those countless taverns were, in their essence, the same church confessional; sitting on a barstool instead of kneeling on a prie-dieu, he murmured the same admissions to strangers who were not priests and could not give him absolution.
  


• rataplan: the iterative sound of beating. The night was silent except for the incessant rataplan of the rain.
  


• cicatricial: relating to or having the character of a scar.
  


• keloid: a thick scar resulting from excessive growth of fibrous tissue. How many new scars if he survived - how many pale and puckered cicatricial welts or red keloid monstrosities from hairline to chin?
  


• coruscate: to gleam with intermittent flashes. Pain coruscated through his legs, weakening him and testing his balance.
  


• jalousied: having horizontal slats. If it had been fixed or jalousied he would have been trapped. Fortunately, it was a single pane that opened inward from the top on a heavy-duty piano hinge.
  


• coelenterate: any of a species of radially symmetrical invertebrate animals including the corals, sea anemones, jellyfishes, and hydroids. Down the middle of the hall is an intricately patterned Persian runner, in which the curved and curled and undulant shapes absorb the radiance of the full moon and glow dimly with it: Hundreds of pale, luminous coelenterate forms seem to be not immediately under my feet but well below me, as if I am not on a carpet but am walking Christlike on the surface of a tidepool while gazing down at the mysterious denizens at the bottoms.
  


• lumpen: of or relating to dispossessed and uprooted individuals cut off from the economic and social class with which they might normally be identified. "I would hate to think," Spencer said, "that you are a dropout, resigned to the status of a lumpen mammal, unconcerned about being exploited, all fur and no fury."
  


• ormolu: golden or gilded brass or bronze used for decorative purposes. Early-nineteenth-century French furniture, with elaborate marquetry and ormolu.
  


• lambent: softly bright or radiant. Under the lambent light of the laser, another print appeared.
  



• reliquary: a container or shrine in which sacred relics are kept. Reluctantly, while stopped at a traffic light, he returned the hand to the container. He put that reliquary and its precious contents under the driver's seat.
  


• philtrum: the vertical groove on the median line of the upper lip. Nothing about the set or width of her mouth, the contours of her philtrum, or the shape of her teeth was even intriguing, let alone electrifying.
  


• glabella: the smooth prominence of the forehead between the eyebrows. Slowly, he moved his finger toward her face as he said, "You. Have. The. Most. Exquisite. Glabella. I. Have. Ever. Seen."
  


• lagniappe: a gratuity of any kind, tip "Yes, I'm sure that without the lagniappe of Ackblom's art, things here would be grim indeed."