The man in black escaped through the desert and the gunslinger followed him.  It is unclear why he does so, it's even not clear if he is a good or a bad guy.  The gunslinger travels through a strange land, dry and a bit mystical.  With only that one goal, to get to the man in black, and he will set everything aside that comes in his way.

Stephen King started writing this in a very early stage in his career, and it took him several years to finish.  Certainly the first few chapters "suffer" from being written by a less experienced writer, but don't let this keep you from reading it.  This is certainly not King's best novel, not even close, but it's an important entry into the magical world of the Dark Tower.

It shows a decaying world, where beings and plants try to survive while knowledge, habits, even manners get lost rapidly.  King is very good at describing an atmosphere, a surrounding, and even as a young writer he excelled at doing this.  No, don't expect page-long descriptions of a dry desert, the power of King's writing is that he hides this describing in everything else, so that the land the gunslinger travels trough unravels for us together with him.

Do I recommend this?  I don't really know.  As I said, it's not his best work.  But it has the first important seeds of what is becoming an epos comparable to Lord of the Rings (in my humble opinion), and for that reason alone it is worth a read.  And a re-read, as I just did ;-)