Dracula by Bram Stoker
When it comes to pre-20th century horror, there it doesn't get much better than Bram Stoker's Dracula. It has been turned into more films than we can count, and very few do justice to how great the original novel really is. The novel is about Dracula, who is trying to relocate from Transylvania to England, and ends up fighting a group led by Abraham Van Helsing. So much of modern horror would exist without this novel that it's crazy.
Scare Factor: 2 - Definitely moments where things get pretty intense, but is mostly wonderful just for its gothicness and great characters.