I'm surprised it hasn't been mentioned before, but....Goosebumps. ;) By R.L. Stine.

Also check out Dr. Jekyll and Mr. Hide, and everything by Stephen King as has been already suggested.

Harlan Ellison. Haven't seen him mentioned yet.
Good horror if you're alright with a brutal style of writing. Even has a game on here on GOG called, "I Have No Mouth And I Must Scream." Cheesy title, but it's really well done. I think you can find the short story free online with a google search.