This is the direct sequel to Off Armageddon Reef, which you should definitely read first. This volume advances the overall story arc and ends at an only slightly frustrating place.
I could find a lot of reasons to say this is a poor book. The whole world is very contrived to create the setting Weber wanted to play in and call it science fiction, and in this book some new details about the history are revealed that border on deus ex machina. The level of gratuitous detail about the workings of sailing ships, cannon, naval battles, and such is down from the first book but still pretty high. Several major plot points, while emotionally satisfying, are just too pat; things shouldn't work out quite so perfectly except at the end of a fairy tale. It is the measure of Weber's writing that, in spite of being aware of all this, I just couldn't put this book down.
While most of the wider story (beyond the interactions of the individual characters) is pretty whimsical, this book reaches for the profound in its examination of religion. In explaining why the bad guy's version of the world religion is evil, but the good guys really are good guys and really are motivated by genuine faith, it comes pretty close to articulating how I feel about organized religion.
A lot of people will probably argue that this book doesn't deserve to be called great. It does have real weaknesses. But it grabbed me so hard that I can't give it less than a 9 out of 10.
**** PLOT HIGHLIGHTS -- SPOILERS ****
At the end of the last book, Charis has pretty well achieved global naval supremacy by wiping out all of its enemies' navies, but it's still one small country against the world and its long term position seems pretty hopeless. This book covers several happenings that improve Charis' position. These things are actually somewhat intertwined but I'll pull them apart.
The most important development is that Cayleb manages to secure Chisholm as a solid ally. They were reluctant participants in the ill-fated war in the last book, and while giving essentially no quarter to the other powers, Cayleb returned Chisholm's surviving ships without conditions. He then presents his argument that if the Group of Four gets away with killing Charis, Chisholm will be next in their sights. Against this backdrop, rather than a simple alliance, he proposes merging the two countries into a single Empire of Charis, and to seal this, proposes marriage to Sharleyan. Sharleyan accepts Cayleb's argument of political necessity and agrees, and then the two of them turn out to hit it off perfectly.
The next big thread is that Naarmahn of Emerald realizes that he's going to be completely toast once Charis gets around to taking over his country, so he tries to negotiate terms. Cayleb, in a surprising show of wisdom (not that he's a dumb guy), decides that Naarmahn can be managed and be very useful, so he not only spares his life, he leaves him as Prince of Emerald, with Emerald nominally equal to Charis and Chisholm in the newly formed Empire of Charis. Using Merlin's intelligence, Cayleb gives Naarmahn extremely fine details of his past activities by way of demonstrating that if Naarmahn ever acts against Cayleb or Charis, they will know and they will show no mercy. Then he puts Naarmahn in charge of external intelligence, a position where his scheming mind will have plenty to do and won't be tempted to turn traitor.
In another thread, the Group of Four demonstrates its increasingly bad leadership by ordering the kingdom of Delfarahk to seize all the Charisian ships in port. The Inquisition agents involved deliberately provoke an atrocity. Cayleb doesn't want to be distracted from the real business at hand, but dispatches a fleet to level the city anyway. This will provoke the Group of Four to attempt to close all of the world's ports to Charis, a tactic which is almost sure to backfire. With Charis in total control of the seas and essentially in total control of merchant shipping, cities all over the world will be choosing between violating the embargo and facing complete economic collapse.
Erayk, the formerly corrupt Archbishop of Charis, has rediscovered real faith in his cell on death row. At his public execution for heresy, he denounces the Group of Four, ensuring that he is actually gruesomely tortured to death rather than being quickly strangled before the prescribed mutilations are carried out. His wife and children escape to sanctuary in Charis.
An obscure old monastery on the outskirts of Tellesburg turns out to be the headquarters of a secret cabal, the inner circle of which knows the truth about the founding of Safehold and how the Church of God Awaiting is founded on lies. We learn that the reason that Charis is so theologically liberal is not only that it's far away from the Temple, but also that a whole lot of important clerics for the last few hundred years have been trained here. Several of the important good guys are initiated into this inner circle. The inner circle itself must vote to approve each person who will share the secret, and it's a major point of dissension that they won't allow Cayleb to tell Sharleyan.
At the end of the book, Cayleb is off with the fleet to kick Hector's ass, leaving Sharleyan in charge of the empire.