First published in 1796 (after being written over a ten-week period by 20-year old Matthew Lewis), The Monk is one of the most gripping 18th-century novels I've ever read. Taking the Gothic narrative devices introduced by Horace Walpole and Ann Radcliffe and stretching them to their most graphic and horrifying extremes, Lewis' book is a ghastly tale featuring fallen priests (and their subsequent depravity) mistaken identities, uncanny predictions, bloody riots, violent plots and subplots aplenty, a fair helping of ghosts, and even the devil himself. Even today some of its language would be considered fairly extreme - in its own day it was, as Coleridge wrote, a book "which, if a parent saw it in the hands of a son or daughter, he might reasonably turn pale."
Fascinatingly digressive (several long stories, usually having something to do with ghosts but having to do only tangentially with the main plot, are recounted in the middle) and at times quite funny (when it's not being wincingly disturbing), The Monk is not quite like anything else I've ever read. Some elements of it have reappeared in countless Gothic stories since, but its preeminence in the field deserves wider acknowledgment.
I read the Oxford World's Classics edition, which was perfectly acceptable except for the fact that while certain passages were glossed with asterisks, there were no notes or annotations to be found anywhere in the volume: an unfortunate omission indeed from a fascinating volume.