You're aware, no doubt, that Uncle Tom's Cabin was the best-selling book in America, other than the Bible, during the Nineteenth Century. But what work held that honor before Harriet Beecher Stowe's novel displaced it during the 1850s?
The surprising answer, according to David Goldfield's America Aflame: How the Civil War Created a Nation, is a book entitled Awful Disclosures of the Hotel Dieu Nunnery of Montreal, purportedly written by one Maria Monk and originally published in 1836:
Maria Monk's problem was . . . [that] she was pregnant in an era when unwed motherhood was decidedly unfashionable. The solution: write a book,a strategy concocted by a team of prominent evangelists and abolitionists, including Theodore Dwight . . ..
Awful Disclosures, true to its title, chronicled the debauched life of nuns and priests in a Montreal convent. Mother Superior parceled out nuns to priests and issued orders for the murder of their babies. Licentious priests roamed the halls, feasting on young virgins at will. "Often they were in our beds before us." Maria wrote. Especially fascinating for readers were the detailed descriptions of the convent's Gothic rituals. The ceremony marking Maria's entrance into the novitiate required her to lie down in a coffin, after which three priests ravished her (accounting for her pregnancy). Mother Superior forced her to reveal her most secret thoughts and desires to priests in the confessional. Maria concludes her story with a tour of the convent for the reader, probing the secret recesses and passageways down into a subbasement where she discovers an enormous lime pit employed to devour the bodies of the murdered infants.
Now, the idea of exploring early 19th Century porn naturally aroused my interest, and sure enough I found a copy at Google Books (above, I've linked to a second edition, also published in 1836). Alas, the porn is pretty thin gruel, so vague as to deprive the passages of titillatory (I made that word up myself!) character (some paragraph breaks added):
Nothing important occurred until late in the afternoon, when, as I was sitting in the communityroom, Father Dufresne called me out, saying he wished to speak with me. I feared what was his intention; but I dared not disobey. In a private apartment, he treated me in a brutal manner; and from two other priests, I afterward received similar usage that evening. Father Dufresne afterward appeared again; and I was compelled to remain in company with him until morning.
* * *
Some of the priests from the Seminary were in the nunnery every day and night, and often several at a time. I have seen nearly all of them at different times, though there are about one hundred and fifty in the district of Montreal. There was a difference in their conduct; though I believe every one of them was guilty of licentiousness; while not one did I ever see who maintained a character any way becoming the profession of a priest. Some were gross and degraded in a degree which few of my readers can ever have imagined; and I should be unwilling to offend the eye, and corrupt the heart of any one, by an account of their words and actions.
Few imaginations can conceive deeds so abominable as they practised, and often required of some of the poor women, under the fear of severe punishments, and even of death. I do not hesitate to say with the strongest confidence, that although some of the nuns became lost to every sentiment of virtue and honour, especially one from the Congregational Nunnery whom I have before mentioned, Saint Patrick, the greater part of them loathed the practices to which they were compelled to submit by the Superior and priests, who kept them under so dreadful a bondage.
When the topic turns from sex to infanticide, however, the book becomes far more specific and lurid:
It will be recollected, that I was informed immediately after receiving the veil, that infants were occasionally murdered in the Convent. I was one day in the nuns' private sick-room, when I had an opportunity, unsought for, of witnessing deeds of such a nature.
It was, perhaps, a month after the death of Saint Francis. Two little twin babes, the children of Sainte Catharine, were brought to a priest, who was in the room, for baptism. I was present while the ceremony was performed, with the Superior and several of the old nuns, whose names I never knew, they being called Ma tante, Aunt."
The priests took turns in attending to confession and catechism in the Convent, usually three months at a time, though sometimes longer periods. The priest then on duty was Father Larkin. He is a good-looking European, and has a brother who is a professor in the college. He baptized, and then put oil upon the heads of the infants, as is the custom after baptism.
They were then taken, one after another, by one of the old nuns, in the presence of us all. She pressed her hand upon the mouth and nose of the first, so tight that it could not breathe, and in a few minutes, when the hand was removed, it was dead. She then took the other, and treated it in the same way. No sound was heard, and both the children were corpses.
The greatest indifference was shown by all present during this operation; for all, as I well knew, were long accustomed to such scenes. The little bodies were then taken into the cellar, thrown into the pit I have mentioned, and covered with a quantity of lime.
I afterward saw another new-born infant treated in the same manner, in the same place: but the actors in the scene I choose not to name, nor the circumstances, as every thing connected with it is of a peculiarly trying and painful nature to my own feelings.
If you want a book featuring murderous monks, debauched nuns and violated virgins, complete with special guest appearance by Satan himself, I'd recommend instead Matthew Gregory Lewis's 1796 novel The Monk. I won't tell you that I pulled out my copy and confirmed that I bought it almost forty years ago.