This is the first book in a new series, introducing a new world. It ends at a reasonable stopping point in a story that clearly continues.
The starting premise of this book is a rather unpleasant one, and the central character is someone I can't truly believe in. A short book's worth of plot, much of which is inevitable, is stretched into a very long book, in part due to many meticulously described battles. From that, you might expect that I hated the book, but I can't say that; it is clearly written by the same David Weber that brought us the Honor Harrington books, and it's fun to read despite the flaws. It is, however, definitely too long, and not quite as compelling as the Honorverse.
7 out of 10.
**** PLOT HIGHLIGHTS -- SPOILERS ****
The opening of the book shows us the last human fleet being caught by the implacable alien Gbaba and executing their planned ruse to try to sneak a human colony away fro the alien's detection. The ruse appears to be successful, but we assume (though we don't see it in the book) that Earth has been wiped out and this colony is the last trace of humanity. Now fast forward 800 years. At the founding of the colony, a faction of the crew lead by Langhorne got control and implemented a plan under which Langhorne and his cronies were gods and the population was under the absolute control of an oppressive religion which guaranteed that humans wouldn't develop technology that would attract Gbaba attention -- by suppressing technology and inquiry. Our central character, Nimue Alban, wakes up in a robot body (called a PICA) and comes up to speed, learning that her mentors had been on the losing side of the struggle with Langhorne and had gotten nuked in the formative years of the colony. (The site is the Armageddon Reef of the title.) She is determined to rescue the population from the legacy of the nasties. In the male-dominated medieval society, she would not get far as a woman, so she adjusts her robot body to male, takes on the name Merlin Athwares, and plans to insinuate herself in the government of Charis, the most forward looking country on this benighted world. She/he accomplishes this by saving Crown Prince Cayleb from an assassination and assuming the identity of a seijin, which is something like a saint with super powers. She/he uses the the intelligence she gathers from her spy satellites and drones as the basis for "visions". First she/he roots out all of the spies in Haarahld's court, including the king's cousin the Duke of Tirian. The king's first minister tries to confront Tirian on his own and has to be rescued by Merlin. Then, having taken care of the spies, Merlin gathers up the smart people in the kingdom and starts feeding them new ideas. He introduces decimal arithmetic and the abacus, schooner rigging (the world has sailing ships, but they are limited to simple square riggers, and the primary naval vessel is the galley), and several improvements to the primitive cannon the navy is armed with. Charis is very far from the seat of power of the Church and already distrusted, but the new ideas flowing out of it cause the Group of Four who run the church to decide that it has to be eliminated. The church directs Emerald and Corisande, enemies of Charis and each other, to unite with a couple of other countries. Merlin's new weapons, however, are powerful enough that Charis instead destroys all of the enemy navies, so at the end of the book they have total naval supremacy despite being one small country against the whole world.