When I read the first volume of this trilogy, I was dying to read the second and third. However, the second was somewhat of a disappointment. Williamson isn't good at carrying a GOOD story across multiple volumes. It started dragging and I could begin to anticipate what was going to happen next. I'll read the third, just to round out the series.
I think you have to be a complete, dyed-in-the-wool Williamson fan for this to be an exciting series. I DID like the first book. And I loved his short story in The Big Book of Ghost Stories (which is what encouraged me to read this series). If I read more of his work, it will be other short stories.