The Joke:
I'm reading a horror book in braille.
Something bad is about to happen.. I can feel it!
The joke is a pun on the word "feel."
In a horror story: "I can feel it" usually means you have a scary gut instinct that something bad is coming.
In Braille: You "feel" the words with your fingers to read them.
The punchline works because the reader is literally touching the scary parts of the book.