What made this series unique at the start was the concept of trying to survive an apocalypse by living in the sky aboard airships. Once the focus moved down to the planet for the last two books there were times when the series felt like it had gone on a bit too long. The same storm related issues continue to arise over and over and the fights with the sirens have gotten old, but each time the story starts to drag a bit it is the characters that save the day. After a slow start to this one I was once again heavily invested in the outcome before it was over due to the characters and my desire to see them succeed. King Xavier has more enemies to fight than ever before, and that is saying something, and that means that humanity's future is as bleak as it has ever been. Certain Cazadores still resist his rule and cause internal strife while even larger external threats are organizing to come and take the Vanguard Islands for themselves. Once more we get a chance to find out if X, aka The Immortal, can live up to his nickname, or if time has finally run out for the dream of living on the surface.
X is still reeling from the loss of General Rhino and he is haunted by the decision he made to exile Aida, which results in all of that stress driving him back to the bottle. Things go from bad to worse as he attempts to drown his sorrow and those around him can see that his behavior is putting them all at risk. Without X it will all crumble quickly so nobody wants to push him past the point of no return but if he doesn't come around they will all pay the price. While these relationship issues try to sort themselves out those who want to see X fail start putting their own plans in place to take advantage of the situation. It all builds to a climactic conclusion that made this one of the better book of the series.
I have no idea how many books are planned for the series but I wouldn't be sad to see it end here on a high note as the ending makes for a good stopping point. I have enjoyed this series way more than I thought I would and I don't want to see it overextend itself. I also would love to see a TV series and video game made out of this material and I hope they go in that direction. As far as the audiobook version goes, R. C. Bray is the perfect narrator for this and his performance as X makes this a great series to listen to.