"The Dark Tower" series is arguably Stephen King's life work. He set out writing the stories that ended up in the first volume in 1982. I don't think he realised until much later that the seed planted in those stories could be developed into a much richer series of books.
The series, unfortunately, isn't finished yet. Last year saw the release of the fourth volume, in which protagonist Roland the Gunslinger still hasn't fulfilled his quest of finding the elusive Dark Tower. King claims that another two or three volumes may appear before the Dark Tower is found.
The first volume, a none too voluminous book called "The Gunslinger", isn't too good. The landscape is difficult to identify with - a world which intersects in some ways with ours, but is often also totally different. It functions as an introduction to the character of Roland of Gilead, Son of Steven, a.k.a. The Gunslinger. He follows the Man in Black. It's a pretty standard story, with little character depth.
"The Drawing of the Three", still set in a surreal, nightmarish world, is already a lot better. In order for Roland to find the Dark Tower he has to form a ka-tet - a bond of friends, sortof - or else he'll fail. Through doors on a dangerous beach he looks into various timeslides of our world and 'draws' the people he needs: The Prisoner and the Lady of Shadows. Here, King starts to develop the characters a lot more, creating easier identification we all know and love from his best works. Roland isn't perfect, you see. And the people he 'draws' into his world are also multi-facetted. The book is full of suspense and, with a little suspension of disbelief, possibility. Great.
Third in the sequence is, as far as I'm concerned, his finest book in the series yet - "The Waste Lands". Roland, Susan (the Lady of the Shawdows) and Eddie (the Prisoner) set out across a very imaginative landscape with weird perils, odd people and great dangers such as only the best of narrators can conceive and describe. They now have to 'draw' Jake, the boy who in "The Gunslinger" gets sacrificed for Roland's quest. A powerful story line with great narrative interlocking, almost as if the book was first conceived as a puzzle. Jake becomes not just "the boy" he was in "The Gunslinger", but again a credible and three-dimensional character. By the end of the book they head out from Middle World to End World using a monorail that's dangerously alive and on a suicide course... It is difficult to describe why this book is so good, but it just is. It justifies reading both previous volumes, and the second was really good on its own accord as it is.
Using the same characters in a time-telling frame this time, the fourth volume ("Wizard and Glass") is basically a huge flash-back of a time 'before the world moved on'. A beautiful love story with the odd detective ingredient thrown in, intermingled with Stephen King's horror of course. Well written and quite enjoyable to read, the book is a bit of an odd-one-out in the series. Still, very enjoyable. I always was a bit of a sucker for love stories, which this volume is, at its heart.
All in all, "The Dark Tower" series should be counted among King's finest, together with "It", "Needful Things" and, of course, "The Stand" (then again, I should add "The Shining", perhaps, and "Christine"...the list goes on).