I bought this years ago in order to read Little Sisters of Eluria, a short story featuring Roland the Gunslinger from the Dark Tower series. It was years later when the story made its way into Everything's Eventual. I've never read the other stories.
The plot of Little Sisters is fairly straightforward. Roland gets injured fighting some mutants and is taken to a convent so his wounds will be treated. Only, there is much more to the convent than meets the eye.
If all you're looking for is the Dark Tower story, I'd go with Everything's Eventual instead. That way, you also get the story of Dinky Earnshaw, one of the Breakers.