King of the Universe

With The Dark Tower books by Stephen King finally being turned into a major motion picture, I thought it was time I dove into them. For years I wanted to read them and finally I am. In the second book, The Drawing of the Three, I came across an interesting easter egg that got my mind racing! 
I'll make sure to keep it spoiler free for those of you who haven't read the series and are planning to. A character from a differen't world than The Gunslingers makes a reference to a very popular movie made by Stanley Kubrick. It was crazy because the particular movie he talks about is The Shining. That is, of course, a Stephen King novel! 
What's the big deal with this? After all, King makes references to his other work all the time! 11/22/63 mentions Pennywise, The Dead Zone talks about Carrie's high school, and countless others. Well, this is different. Stephen King makes mention of the movie The Shining. This leads me to believe that Stephen King is alive and writing books in his particular universe! 
Now, here's where I get a little confused. If Stephen King exists in the universe in which this particular character, Eddie, is from, does that mean Eddie is from our universe? Does that mean all of the other books King has written exists in their own universe? Is there a door in which Roland could step through and meet characters such as Johnny Smith, Carrie White, or even Christine? 
And on a different note, from the first book, does that mean the boy Roland befriends is from our universe or King's? It seemed as if the boy comes from Eddies universe, which would be ours as well. When Roland stepped through that door he actually stepped into the real world, our world. The stories of The Shining are just books and movies. But the boy, Jake, is from the universe King created. That means he shares a world with Dale Barbara from Under the Dome and Danny Torrence from the Shining! 
I'm sure this has all been talked about before. I'm a little late reading these books but I still found the concept fascinating. What about you? Have you read The Dark Tower books? What do you think about this shared universe concept? I'd love to hear about it! Let me know in the comments below. As always, thanks for reading!