The book: An Autumn War by Daniel Abraham.
This is book 3 in the Long Price Quartet. I bought the first two in an omnibus edition from the Science Fiction Book Club. In fact, their non-offering of the remaining two books as an omnibus is one of the things that lead me to stop being a customer of theirs. But that's a rant for another day. I read the first two books and enjoyed them greatly, and never got around to reading the last two. Until recently, when a paperback omnibus edition was released. I requested the volume for Christmas and was fortunate enough that Mrs. S. gave it to me over the holidays. Since it had been several years since I'd read the first two books, I re-read them to get ready for book 3. They were very good, much as I remembered. But looking back at my book log, it has taken my over 2 weeks to read An Autumn War. Normally I'd read 5 - 6 books in that time period. The book was good, but painful to read. If I weren't such a completist when it comes to reading books, I'd have put it down and moved on long ago. As it is, I can't bring myself to read book 4 yet.
So what happened? Let me put forward a few ideas. First off, the book removes the very unique magic system from the world, as the main premise of the plot. The culture portrayed in books 1 and 2 is the sole possessor of this magic, and when it goes away, they are vulnerable to military conquest, having trusted in their magic to keep them safe for generations. So, no cool magic = less cool story. Next point: the book is about the attempt to militarily conquer the now defenseless society, yet the military stuff is all off stage. Whatever gratification there is to be had from reading descriptions of battles is not present. Mostly what we get is the POV of the leaders of both sides of the conflict, feeling guilty about doing things they feel they must, in service of the greater good. A lot of talking, but not a lot of doing.
Anyway, I wanted to find out what happened to the characters, and I'll eventually read book 4 for that reason, but this book just didn't hold my interest as much as the previous two.