The premise of Fairy Tale (person from our world crosses over into alternate world/ reality/ universe) is one that King has used many times over the years – most successfully in The Dark Tower series (although in that case it was people crossing every which way across worlds/ realities/ universes).
The protagonist of the book is a high schooler named Charlie Reade whose mother died when Charlie was young and whose father, as a result of the death, fell into alcoholism. The book details much of Charlie's upbringing and family backstory as well as how Charlie ends up forming a relationship with the neighborhood hermit, Mr. Bowditch. Eventually Charlie learns that Bowditch isn't exactly who he seems and that he actually has a portal to the aforementioned alternate world in his backyard (a lot happens in between all this, but this is the very high level direction of the book).
I really enjoyed the "real world" sections of this book and thought King did a great job creating the characters of Charlie, Charlie's father, and Mr. Bowditch. The sense I had reading the book was that King really liked these parts of the book as well, because they take up far more pages than one would expect given the overall premise of the book (about half). For instance: imagine if it took us an hour of the movie before Dorothy got to Oz. Such proportions would just feel off and would in many ways discount the core of the story, which, of course, is Dorothy's adventures in Oz.
Once Charlie gets to the eponymous Fairy Tale world, I actually grew less interested/ engaged with the book – and while there were still some good scenes – I struggled to stay engaged with the story. To tell the truth, I was much more interested with how Charlie might interact with his father once he returned to the real world, vs. any of the fantastic events in Fairy Tale Land.
In the end, Fairy Tale is a book with some strong aspects, but as an overall work felt pretty flawed and is not among King's best efforts.