It is a complicated book, particularly since it follows five distinct groups through the turmoil that ends the year 1633. It follows Rebecca Abrabanel and the delegation to France and the Netherlands, Rita Simpson with Melissa Mailey and the delegation to England, the beginning of the US Air Force, the new US Navy with their soon to be completed ironclads, and the political machinations of Mike Stearns. And that's just the major plotlines.
Another point of interest about 1633 is that it, unlike 1632, is explicitly imagined as part of an ongoing project involving far more authors then just Eric Flint and David Weber. That project has produced the "Ring of Fire" anthologies, as well as the continuing Grantville Gazette e-magazine. Occasionally that understanding leads to some interesting choices in this book. An example is the brief scene between Mike and Harry Lefferts, where Harry promises to get Anne into Amsterdam. Another example is the ironclads. By the end of the novel they still haven't been floated.
I enjoyed it tremendously. Its complex, as real history generally is, with a great many interesting and vivid characters. There are plenty of machinations to be considered. There are also several cases of Americans showing off the power of modern know-how and/or technology. In one case they chase off some pirates, in another they use a small fishing boat to sink a Spanish galleon.
This book, like 1632, can be read online at the Baen Free Library.