This is a direct continuation of events begun in the second book of this series, Exiles at the Well of Souls. These two books, together, are called the War of the Well World.
It is difficult to describe the events of this book without substantially spoiling much of Exiles.
Essentially, the second phase of the war gets under way, and I can't say any more than that.
As the author puts it in his note in both books, "In an ideal world, Exiles should be read first."
If you have read Exiles, and found it a good book, this one is a must (it's really the second half).
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