tl;dr: It's David Weber. FTWLTSOT,TITSOTTL1
I finally slogged through the entirety of David Weber's Safehold series. "Slogged" is exactly the right word. These books are doorstops bar none. I vaguely guess there's somewhere between 3 million and 3½ mililon words total in these nine books, most of them clocking in somewhere between 700 and 850 pages. They're so full of David Weber tropes it's almost like Weber fanfic: war, blood, heroism, swashbuckling derring-do, angelic heroes of the most upright standing and mustache-twirling villains of foregone depravity.
It's not a spoiler to reveal the setting and basic plot: It is The Future. Humanity has all but been wiped out by mindless alien xenophobes, and Safehold, the last human colony has been hidden far away from the interstellar warfront. To make it even more invisible, the colonists have been mindwiped and dropped into a 15th century tech level with a book, The Holy Writ Of Langhorne and the Archangels, which explains how God wants them to live: healthily (the books explain everything: animal husbandry, antiseptic would care, terraforming, even some cognitive psychology) but simply (there are proscriptions against invention and innovation, with absolutely hideous death-by-torture punishments for anyone daring to experiment with electricity, backed up by an automated orbital death machine that will rain down Mjolnir-level death on any city that starts to use electricity extensively), the better to avoid putting out the radio and neutrino signals that the aliens used to track Terran vessels down. It's a clever conceit that lets Weber have his beloved sailing ships and broadside-to-broadside battles, but it reads like "Hey, what if someone wrote Honor Harrington but like, on water, with sailing vessels?"
A thousand years later, into this mess awakens Nimue, the last surviving woman of the Terran Federation. She's been reincarnated in a robot body (Weber does a lot of handwaving about posthuman technologies to explain why H. sapiens hadn't NerdRaptured™ to deal with the aliens), and learns that the "hideous torture" part wasn't actually in the original plan. There was a coup among the colony commanders, and the "No tech, ever" team murdered the "We'll have to deal with the aliens eventually" team. She formulates A Plan to invalidate the Holy Writ, and touches off a massive World War™ between the liberal (read: England and the better, more Protestant, parts of Europe) and conservative (read: The nastier, more Catholic, parts of Europe, plus a few Chinese thrown in for good measure) parts of the planet. Without using electricity, Nimue bootstraps the most liberal political entity (the one that had banned slavery and adopted universal literacy, etc. etc.) on the planet from rowed galley ships to, well, that would be spoiling.
The problem with the Safehold books is twofold. The first is what David Brin calls the spearcarrier problem. Safehold is a Great Game story. A few people in positions of power send millions out to die in their battles. We get names of many soldiers and sailors who get killed within one or two chapters, with details about their wives, friends, parents, and children thrown in to remind us that these are real people who get killed when the Great Gamers start to roll the dice. They still get killed nonetheless.
The second problem is one that's familiar to readers of long series from Weber: While we want the heroes to win, we don't want the winning to be easy. The winning on Safehold is easy. The main characters are rarely, if ever, in any real danger of losing. Nimue is an immortal, nearly indestructible, well-trained killing machine who can provide satellite-based, world-encompassing reconnaissance, complete with audio and video recordings of (almost every) conversation the enemies are having to her allies, and who has a very large library stashed away in her Fortress of Solitude, including all the war history and every technological improvement she can supply. Most of our World War One was fought without electricity, after all, and by the end of the series her generals and commanders are quoting Patton and Clausewitz at each other. Despite the overwhelming numbers of soldiers the bad guys have, the good guys always pull out a technological advance that wins that day.
Weber is reluctant to kill his angels. Almost none of them die, all of them have the author's hand of forebearance atop them. The main characters of the first three or four books become distant givers-of-orders, and the last five books are a painful series of watching ordinary people go out and get killed for them.
One thing that might turn off people is how Weber uses religion. He is a Methodist by training, and his characters engage in a lot (and I mean a lot) of talking about What God Wants. The God of Safehold is a deliberately twisted, distant syncretic deity meant to supply the post-mortem muscle behind what the coup survivors wrote in the books of their Writ (the major leaders of the coup: Langhorne, Bedard, Schueler, Chihiro, etc., all have their own books in the Writ), and Weber's point is that "the real, human knowledge of God will shine through even their twisting of it," as the first Archbishop who comes to know Nimue and the true origin of the Writ says while retaining his faith. Nimue in passing mentions that she is a Christian, but it's never belabored much, and she says she'd be thrilled if she could get past all the brainwashing and let all Safeholdians know about Christianity, Islam, Buddhism, and all the rest. It isn't badly handled; in fact, I rather enjoyed how deftly Weber made his Case For God without ever once having to rely on supernatural intervention as a reason to believe.
If you like Weber, you'll probably like Safehold. If you don't like Weber, you probably shouldn't bother.