The Dark Tower series of novels by Stephen King is perhaps best appreciated by fans of the science fiction and fantasy genres, rather than the horror genre for which King is best known. It's an ongoing story based in a sort of alternate universe which is an amalgam of the American Wild West, fantasy, and your standard post-apocalyptic society. The story itself was inspired by Robert Browning's poem "Childe Roland To The Dark Tower Came".
The lead character in the Dark Tower novels is Roland, the last gunslinger in his world who is on a quest to reach the Dark Tower. Roland's world has become a moving ocean of chronal and geographic instability, where one night can become long enough to share an entire novel of information over a campfire, and a desert that took days to cross one way may take months to cross on the return trip. Instabilities in spacetime also exist as portals to other Earths, including our own twentieth century world. It's all (very deliberately) confusing, but you get used to it after the first book or two.
Stephen King has often made references to certain of his novels in other novels -- especially in those books which all take place in the town of Castle Rock, Maine -- but the Dark Tower series seems to be setting itself up as the center of them all. Like the Dark Tower itself, these novels are a sort of linchpin in time and space, bringing all those separate universes and supernatural events together. At one point in Wizard and Glass, the characters all cross squarely into the world ravaged by biological catastrophe in King's novel The Stand. At another, a character pulled from "our world" makes a reference to the movie of The Shining. A major character in Wolves of the Calla originated in his early novel Salem's Lot. King's earlier novel Eyes of the Dragon also ties in closely to events in this series. The reasons for this are uncovered in the sixth and seventh books.
A notable peculiarity of the first four novels is that they were all released in very limited hardcover formats, and then widely as large softcover books. (The last three novels were released in hardcover formats first.) Both hardcover and softcover formats include several color plates featuring professional artwork depicting scenes from the story.
The Dark Tower novels published are, in order: