This is book 4 of the Safehold series. It will make more sense to start at the beginning, but it probably provides enough recap to pretty much make sense on its own. It tells a complete episode, but still leaves the larger story in a very tense position.
This is a David Weber book. In the unlikely event that anyone reading this hasn't read any Weber, go dig up a copy of On Basilisk Station (Honor Harrington book 1); the Honor Harrington series is his best work. Like most of Weber's books, this is highly military, spending a lot of time examining the politics, intrigue, and human relationships that lead to wars and a fair bit of time in describing actual battles in fairly gruesome detail. It's too long, switching between too many viewpoint characters, and it gets uncomfortably graphic. It's also based on a rather hard to swallow initial premise and skates along on some fairly shaky ground to keep the world as premised from collapsing. People who don't like Weber, or who don't like this series, can make very long lists of bad things about this book, this series, or Weber's writing in general, and they're right in all the particulars. But there is still something about the stories as he presents them that makes them hard to put down, and this book has that quality. There's a lot of background infodump about how sailing ships, cannon, and naval strategy work, but it's actually less extreme than the last couple of books in the series. And, amid all the bluster and excitement, it manages to have something to say about the human condition.
8 out of 10.
**** PLOT HIGHLIGHTS -- SPOILERS ****
I'm not going to try to cover everything; there's way too much. But I'll hit a few bits.
Clyntahn full reveals himself in this book as a completely evil psychopath, even farther than before. He knows about the Circle, and he slowly closes the noose around them, while they're too trapped to escape. When the penny finally drops, Hauwyrd Wylsyn kills his brother Samyl with a sword since he won't suicide, and then does a suicide-by-cop on the squad sent to arrest him. He does this to protect Anzhelyk, as well as to avoid death by torture. 30 of the Vicars, dozens of archbishops and bishops, and all of their aides and families are swept up and tortured to death. Madame Anzhelyk reveals that she's more competent than we knew; she's been amassing wealth and preparing escape routes for decades, and she escapes with a couple of hundred wives and children of the condemned, using Clyntahn's own corruption against him by claiming that her shipping companies belong to him so nobody will check what's in her cargoes.
The good guys win the hearts and minds of Corisande, which started out very much against them because Clyntahn murdered their prince in such a way that it looked like Cayleb did it, through iron discipline and determination to be gentle conquerors, while allowing the bad guy insurgents enough rope to hang themselves, and then using Merlin's intelligence to sweep up the leadership in coordinated mass raids.
The first big naval action sends a fleet halfway around the world to harry the Gulf of Dohlar and make it harder for Earl Thirsk to build his fleet. Unfortunately, he's a very able commander despite being on the wrong side, and the good guys get caught in a really bad storm after they've already decided it's time to get out of Dodge, and for the first time in the series, Charis loses a fight at sea.
In the last big political action in the book, Charis manages to convince Tarot to switch sides almost without firing a shot, and with that, they have pretty much all of the world except for the main supercontinent on their side.
The rest of the book leads up to a big battle, as the bad guys are trying to mass their big fleet. Clyntahn has figured out that they're being spied on, though he doesn't know how, so he makes it semi-public that the fleet is going to go west and actually, secretly, sends them east. Charis has diverted a lot of their strength to meet the expected threat and can't get enough ships to meet the real one. They use skulduggery to cut off a third of the fleet in Desnair, scaring them off with a fleet of merchant galleons painted to look like warships. Their science guys have just invented exploding shells, but they can only barely get a few of them into production in time to get them to the battle. But a very fortuitous storm comes up, allowing the Charis fleet to sneak up on the much larger Navy of God, and through sheer courage and the demoralizing effect of a major new weapon, 25 Charisian galleons manage to completely eliminate over a hundred enemy galleons, which will buy them however long it takes Clyntahn to build yet another navy.