As I've noted before, the Dark Tower series is one of the best I've read. I think Wolves of Calla is a solid mid-adventure story for the series. It's essentially an interruption of the ka-tet's journey along the Path of the Beam toward the tower, as they come upon the town of Calla Ben Sturgis and agree to help them fight a villainous invasion of "wolves" who come to steal the town's children and bring them back to the town "roont", or mentally handicapped.

It's a fun, exciting and engaging story that tells us a lot about the characters without slowing the action. This is probably the only DT novel I could imagine someone picking up as a standalone and actually enjoying. I still think it's best not to, though.