After the events in the first book, the Powers Than Be in Europe have stopped blaming deviltry and witchcraft and accept the fact that Grantville is here and that a new entity, the United States, now exists in Germany. They have brought weapons and technology from the future. They also brought back history textbooks. (And other library books with more information than your average high school history text might actually have.)
Having the knowledge of a possible future changes the course of "present" events. France and Spain, seeing that the results of a pointless, prolonged conflict, skip the Franco-Spanish War of 1635-59 and turn their attention to other targets. Richelieu is still cunning and angling for power.
At the start of the novel, representatives of Grantville are scattered about the continent on diplomatic missions, which go about as well as one could imagine in those troubled times. War is inevitable. However, Grantville hasn't rested on its laurels. It started developing an Air Force (by first reinventing the airplane) as well as a Navy, under the direction of John Simpson, who goes from mere antagonist to major player.
If you liked 1632, you'll probably like this one as well as it takes Grantville and its people to the next step. (If you didn't like 1632, then don't waste your time. If you didn't read 1632 then go back and find it because this one won't make any sense.)
It isn't as standalone as the first one. Obviously, you need the background from the prior book, but also there's a bit of a soap opera feel, making sure that the favorite characters are there, even if there isn't much to do. There are the brushes with historical people and events, which either have to be mentioned/acknowledged as happening or setting up future events. Where the first book had a satisfying finish but was open-ended for the series, this one has a major finale that doesn't disappoint but it doesn't end the story that it started. It's one battle: the war is still going on, and the steps Grantville is planning haven't been taken yet. Yes, I wanted to see the Old World's shock at seeing the full force of the United States Navy, instead of just a few speed boats with guns. (Not that there's anything wrong with that.) Again, look at the cover of the book!
I'll probably take some time off to read other things before getting back to the series. But, yeah, I will probably be back to this series, at least for another book. The major question in this chaotic shared-world series may ultimately be: Just which book is the next one?