This set collects the first three. The Gunslinger introduces the hero Roland, who must reach the Dark Tower in order to save his universe, Mid-World. There are passageways between our world and Mid-World, and a New York City boy named Jake gets shoved in front of a car by Jack Mort ("death"), is killed, and finds himself alive in Roland's world. He becomes Roland's surrogate son.
In book 2, The Drawing of the Three, Roland is attacked by marvelous, poisonous "lobstrosities" and enters our world for help. He takes heroin addict Eddie Dean from 1987 New York and Odetta Holmes from 1964 New York as his team. In a powerful time-tripping scene, Roland confronts Jack Mort and actually changes Jake's Earth history, which has heady implications for Roland's world.
In The Waste Lands, book 3, Roland and company get ensnared in a civil war in the urban waste of Lud, acquire a delightful talking pet named Oy the Bumbler, and find themselves captives of a psychotic train called Blaine the Mono.
The plot is complex, yet weirdly logical. But take warning: this series is addictive, and you may need to also buy book 4, Wizard and Glass. Otherwise, you won't know what happened when Blaine went insane with Roland's gang onboard.