The Dark Tower IV: Wizard and Glass (1997) by Stephen King
I have to say, this wasn't my favorite book in the series.
Granted, it fills in much-needed back-story about Roland's past: why he left his home in Gilead; how he met Susan Delgado; and why he's such a bitter, hardened, haunted dude.
The gist: when young Roland discovers that his mother is having an affair with his father's closest counselor, he decides to challenge his mentor to a duel and thereby earn his claim to manhood. Roland wins the challenge, but his father nevertheless sends him away (with two companions) to the town of Mejis (mostly his father wants Roland out of the way as the political and social problems mount at home).
Once in Mejis, Roland and his friends quickly discover the corruption just beneath the surface, which is connected to the town's collusion with John Farson, a rebel leader (against Gilead). Mejis is important because it is the site of several "ancient" oil derricks, which Farson and his men want to control to power their war machines. Throw in a witch, a pink crystal ball (that's kind of evil), and a bizarre Wizard of Oz scene at the end, and you have the makings of a full-scale drama of massive proportions. There's love, there's magic, there's a weird, shimmery thing called a "thinny" that marks where time/universes are bleeding into one another.
As with the earlier books in this series, I'm most intrigued by King's exploration of time/space and the borders between universes ("whens") decaying, as well as the clash of modern technology and a future (but much less advanced) human society (think TNG's "When the Bough Breaks").
Well, on to Wolves of the Calla.