When the little town of Grantville was sent backwards in time to 1632 and settled in the middle of a Germany fighting the Thirty Years war, the inhabitants of the town had little idea of how their arrival would change the world.
Now it is 1635, and Mike Stearns, the former leader of the USE, the United States of Europe, has been unseated in the election and replaced by Gustavus Adolphus, king of Sweden, He's sent Mike Stearns as a newly created General to battle the forces of Poland, and with him is Jeff Higgins, a newly created Captain who has as little experience with fighting battles as Mike Stearns does. Higgins is annoyed that Stearns is sending someone with zero field experience into battle. Stearns doesn't want to do this, but is bound by his agreements with Gustavus Adolphus to do so.
Meanwhile, Gretchen and Rebecca are dealing with their own problems. Gretchen has decided to write a book on the history of the USE and Gretchen has moved to Magedeburg to work with the Committees of Correspondence and do secret agent-style stuff for them.
But when Gustavus Adolphus is injured on the field of battle and sustains brain injuries, it triggers a crisis of succession, since his only child is a young daughter who is unable to rule on her own, being only nine and a half years old. And although she is promised in marriage to Ulrik, a man many years her senior, there is one man who is poised to bring down the government in the name of strengthening the USE. And that man is Axel Oxenstierna. But if he throws over the king because of his injuries, what sort of government will he bring in place of the democracy that the USE currently enjoys? And what far-ranging repercussions will that have for the USE, the town of Grantville, and the people who live in the area that the USE controls?
Despite the series being mainly the child of Eric Flint, this series, known as the Ring of Fire, is really a shared world story, much like the old Thieves Guild books. Eric works with a number of other authors to flesh out various sections of this new world history, and each writer brings something to the table. Nevertheless, this book reads like a preparation volume for books to come. Something bad happens during the fighting, and now other authors are going to write on how this affects their own parts of the shared world.
And it also sets up political situations for the future. So many other principalities and nations are affected by the USE and what is happening with them that it is like a stone thrown in a pond. Little ripples lead to bigger ripples that will eventually affect the world as a whole. Not just in technology, although that is certainly one area in which the Grantvillers have the advantage. But after three years, they are running out of things that power their uptime conveniences and the ammunition for their guns. Luckily, they are getting close to replicating it by working with the Germans who are part of the USE. And that is also bad, because it means that their adversaries can commit industrial espionage and get the tech themselves.
If you like alternate histories and 'what if" mixed with a dose of reality, this is the series for you. Eric works with enough other writers that, while the entire series may not be to your taste, you can find something to interest you. Most people enjoy the original Eric Flint books the most, but at the same time, many of the alternate or side stories are necessary reads to figure out who these people are and what is really going on. I like this series, but I will admit that I haven't read all of them, and some of the books are better than others. Recommended as a series. Highly recommended for the ones written by Eric Flint alone. YMMV on the other books.