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I can point to a lot of good starting places. Perhaps the best gateway text has proven to be the author's 80s anthology series, Skeleton Key. It's easy to see why this simple collection is often cited as an ideal reading primer. Most of the stories in it can be taken in at one reading, which is a value if the daily schedule is busy. Another plus is that all of them appear to be simple enough in terms of subject matter. In addition to all this, a response I keep hearing from readers, one that seems to span the passage and arrival of generations, is that for a series of unconnected short-stories, the whole thing almost reads like a novel.
Skeleton Key seems to be one of those books that can sometimes grow on the reader. The first time you read it, what grabs your attention are the situations to be found in each individual story, and all the gory special effects that come with it. Those who choose to have a second and, maybe, with any luck, third read-through will perhaps find themselves focusing more on the character dynamics, and slowly become aware of King's skill at drawing you into his narratives. For those who find themselves turned into dedicated readers by the experience, a fifth and sixth study of Key might just make them aware that King is an actual author, one with legitimate, and above all, literary themes embedded in his writings.
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This is even more of a bonus when you stop to realize that while vast majority of people can read, knowing how to read well is often just as much an art as being able to spin a good yarn. Just like books themselves, being able to read them well is a multi-layered activity. What makes any story valuable is what lies beyond its surface appearances. That's an idea that sounds obvious on the face of it, and an immediate assumption is that anyone can do it. It's true, anyone can read if they truly want to. The trouble is you can't expect a young mind to read any given text with an automatic, sophisticated point of view. The goal of being a good reader is to see just how many levels (or lack thereof) is contained within the pages. In that sense, being able to read well is less a natural ability like breathing and seeing, and more like a hidden, invisible skill that you have to work at for quite a while in order to do it well. Skeleton Key helps in that training by offering itself up as a stepping stone to greater heights and conquests.
Nevertheless, I'd like to offer an alternative place to start making this author's acquaintance. While Skeleton Key is often cited as the best place to begin an acquaintance with King's writing, the fact remains that this is just one staring place out of many. Real life experience points to readers getting hooked by works like Cycle of the Werewolf, The Stand, or even out of the ways novels like The Dark Half. I myself, for better or worse (and I still can't say it's all that bad) got hooked on King by listening to Jeffrey Demunn narrate the author's 2001 book, Dreamcatcher. That's an argument of defense for another time. The point goes back to what I said earlier. Everybody finds their own way into this author's work.
I think a novel like The Outsider deserves its place as a beginner's candidate for a number of reasons. The most obvious point in its favor is that it is a neat examination of the theme of the doppelganger in literary Gothic fiction. King uses this trope in his novel to hold a mirror up to the Dionysian/Apollonian conflict in American society. The other point I can think of is that the novel is something of a neat distillation of a lot of the prototypical settings, characters, and situations that sort of typify the nature of a Stephen King book. In the sense, I think what makes The Outsider a good primer for King neophytes is that it helps ground the new reader into a clear idea of the main subject matter of King's secondary world.