The myriad stories begun throughout the series are brought together here, as plot-threads you didn't know existed are revealed and then woven together (and none of them are dissatisfying cheap tricks, either). Many questions are answered, but not all of them.
The scope of the finish here is the biggest I can remember, and the poignacy of the end is distinct from the triumph or anti-climax at the end of many other major science fiction tales.
I've read this entire series four or five times completely, and it was worth it every time.
Vol. 1: Midnight at the Well of Souls
Vol. 2: Exiles at the Well of Souls (Part 1: War of the Well World)
Vol. 3: Quest for the Well of Souls (Part 2: War of the Well World)
Vol. 4: The Return of Nathan Brazil
Vol. 5: Twilight at the Well of Souls