The first King book I read was The Shining, in 1977 and I was immediately hooked. A literary horror novel! I was astounded and went back and read Carrie and 'Salem's Lot and then read everything that followed. At the time, I was researching and writing my first book, Mayberry, My Hometown. After that book sold to a publisher in 1984, I started thinking about my next book and decided to do the Stephen King Encyclopedia. I wrote to King and he essentially sanctioned it. His assistant, Shirely, who is now retired, helped me over the five years it took to write it, and I was able to interview his brother and other family members for the book. As to why King still fascinates me, it's his powerhouse storytelling abilities. I also write fiction and am ceaselessly impressed with his creativity and output. Plus he's a far more literary writer than the perception of him over the years (this negative interpretation of King is fading, thankfully), and I've always enjoyed his writing.
To read the whole in-depth interview head over here.