Full premise: The town of Grantville is transported to the middle of Europe in the year -- wait for it -- 1632, in the middle of the thirty years war. The Americans immediately start making a mess of the status quo.
I didn't realize it was permanently free or I wouldn't have made a big deal out of it. But I understand why they're doing that. There are several direct sequels (imaginatively named 1633 and 1634) and any number of short story collections by other authors and spins-offs and such. It's become quite the franchise. Make the first one free and get them hooked. Hmm, sounds like a business model...
The book is pretty dense reading in parts, because Flint has either done a crapton of research or already was an expert. The amount of exposition sometimes is a bit overwhelming. Also, Flint is obviously a bit of a unionist. He doesn't just make his characters left-wing, he also does the omnipotent viewpoint thing where he pontificates. Right wingers are clearly identified, and have no redeeming qualities at all.
I still really enjoyed the book overall, simply because I enjoy that type of genre. I also liked The Crosstime Engineer and the Island In The Sea Of Time series.