If the last volume, "Wizard and Glass" showed the birth of a Gunslinger, "Wolves of the Calla" gives us the best look at what sort of role the Gunslingers must have played in the world before it moved on. On their inexorable march toward the Tower, we find Roland, Eddie, Susannah, Jake, and Oy passing through Calla Bryn Sturgis, a town in need of help...the sort of help one can get from a group that only deals in lead.
For generations, the Calla folken have seen their children plundered by the mysterious Wolves, stolen away only to be returned as shells of their former selves. Roland and his ka-tet must find a way to stop the seemingly invulnerable raiders and save the town...and, just perhaps, their own quest in the process.
It is in the Calla where we (re)meet Father Callahan, the priest we last saw in "'Salem's Lot." With his entry--one of my favorite subplots in the series--the threads of the various worlds Stephen King has created begin to not only cross, but weave together as part of the overarching mythology of Roland and the Tower.