I'm going to suggest "The Talisman", which was co-written with Peter Straub. It's a coming-of-age story so there should be some overlap of interest there. It's more of a dark adventure than a horror book, and it ties in with the Dark Tower saga (but only "after the fact", so you can read Talisman without any of the other context just fine), but I read it at about that age and it helped me get into a lot of other King books, and eventually, much broader fantasy, sci-fi and horror.

I also seem to remember that it starts somewhat slow, but once you get through that it's fantastic to the end.