Okay kiddo, have you ever seen those really scary stories like Dracula or Frankenstein? Well, a long time ago, people loved reading scary stories like those but they didn't have the internet or TV to watch them on. Instead, they would read books.
One type of book that people really liked to read were called Gothic Blue Books. These books were small and usually had a blue cover, that's why they were called "blue books". They had lots of scary stories inside, with monsters, ghosts, and other scary things that made people jump.
The stories were usually written by different people and they were sold really cheaply so that lots of people could afford them. People could buy them at a shop or by a street seller.
Gothic Blue Books were really popular in the 18th and 19th centuries in Europe, especially in England. And even though they might not be as popular anymore, you can still find some of these books and read the scary stories inside if you want to scare yourself!