My favorite scary book was Harlan Ellison's "I have no mouth and I must scream." It's not clear whether it's the scariest. At the time I found the book in a used book store, I was about 10 or 11. I bought it because it looked interesting and the title caught my eye. When I first started reading the book, I was disgusted, but I couldn't put it down. I think I read the book all at once.