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The top 20 horror novels were selected from Goodread's Definitive Horror Book List and the data was provided by Zenodo's Best Books Ever Dataset.
Regarding genres: after cleaning to remove genres that were common to every book ('Horror', 'Fiction'), genres that were a combination of two or more genres (e.g. 'Mystery Thriller', 'Science Fiction Fantasy'), 'genres' that described a type of media (e.g. 'Classics', 'Short Stories', 'Audiobooks', 'Adult', 'Poetry'), and oddballs (e.g. 'Gothic', 'Psychological Thriller', 'Historical Fiction'), there were 16 remaining. These remaining genres were collapsed into five main categories: Fantasy ('Fantasy'/'Urban Fantasy'); Thriller ('Thriller'/'Mystery'/'Suspense'); Supernatural ('Supernatural'/'Paranormal'/'Vampires'/'Ghosts'/'Zombies'); Scifi ('Science Fiction'/'Dystopia'/'Apocalyptic'/'Post Apocalyptic'); and Drama/Crime ('Drama'/'Crime').
Regarding ages: When there were discrepancies between the first publication date provided in the Zenodo dataset and the Goodread's date, the Goodread's date was used.