Up until the last three decades of the 20th Century, horror fiction was primarily found in short story form. There were relatively few novels that had a major influence, although those that did were big. A list of the five most influential horror novels of the early days follows. All of these are in the public domain and I’ve added links where they can be found for free online.
1. Horace Walpole: The Castle of Otranto, 1764. Generally considered to be the first Gothic novel. Still surprisingly readable. Online here
2. Clara Reeve: The Old English Baron, 1777. The world’s first horror pastiche. The influence of Walpole’s The Castle of Otranto is clear. Download it here.
3. Matthew Gregory Lewis: The Monk, 1796. Written before Lewis turned twenty and so successful that forever after the author became known as “Monk” Lewis. For the time it was considered horrendously blasphemous and sexy. I think it still reads pretty well today. Download it here
4. Mary Shelley: Frankenstein, 1818. An influence on the development of science fiction as well as horror. Check it out online here.
5. Charles Robert Maturin: Melmoth the Wanderer, 1820. An early “Deal with the Devil” novel. Find it free online here.