Some titles that have not been discussed:
The Raft (short story): Just one of the most gruesome stories I've ever read (where four teens are trapped on a raft by an inexplicable oil-like creature in the water). I just makes my skin crawl to think about, but I can't stop thinking about it: It's very haunting in the way it's written, with some internalized subjective perspective...
The Last Rung on the Ladder (short story): No supernatural element, just a man recalling his childhood experiences with his sister and their jumping game in the barn. Beautiful, bittersweet, it chokes me up...
The Mangler (short story): Just an all-out crazy "the machine's possessed by the devil!" story but just so well done... and again, the unique gruesome nature of the machine itself...
The Langoliers: Great novella, unique idea. Fast-paced, and the CHARACTERS really carry it along. King is best when he focuses on characterization and doesn't get caught up in silly horror elements. His people are very life-like when he's in top form, people I care about easily. The whole thing here with Nick and him piloting the plane back alone, I still remember that vividly...
The Long Walk (Richard Bachman novel): Psychologically interesting, with people forced into a situation out of their own greed/ambition to walk or die. What is the psychological impact, how do people relate to each other, what tempts them, what drives them? Just fascinating... and again, the bittersweet ending.
The Girl Who Loved Tom Gordon: Some of this short novel was cheesy, but the concept was interesting, along with "bear creature" and the subjective fantastical perspective of the girl/narrator.
Pet Sematary: This wasn't his best work, but it was just the perfectly planned plot: The doctor who is terrified of losing patients now and has made a solemn vow to restore life is tempted by the prospect of returning life to the dead, even though in the process he knows deep down he's selling his soul to do so. And, as a parent, the phrase "His cap. Oh God, his cap is full of blood" still haunts me.
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I remember Firestarter (mostly the relationship between Andy and Charlie, and Andy's love for his daughter) hitting me hard, as did the Dead Zone (just the whole tragic nature of Johnny Smith).
IT was the first epic story that really creeped me out, I read it in less than a week while studying for mid-terms at college. It stuck with me a long time, and spurred on ideas for my own writing. And I'm planning on rereading The Stand again, now that I'm older.
One of the things I did always love about King was that he was self-referential to his own stories and had his own "pocket universe." So you never knew who else might show up or be referred to in another story.