In King's collection of short stories, Everything's Eventual, one of the stories is a sort of prequel to The Gunslinger. The story's title is "The Little Sisters of Eluria", and is very much a Dark Tower story. Interestingly, in the brief introduction to the story, King mentioned how hard it was to write something related to the Dark Tower series as "short" -- The Dark Tower always wants to be "epic", he wrote.