1633 is the "official" sequel to 1632 (if such a word makes sense for a book that has spawned half a dozen books (not counting short stories). It teams the prolific Eric Flint with the master of military space opera: David Weber.
1633 picks up where 1632 left off and charts the wider political ramifications of the Ring of Fire. The story primarily charts the action in North-Western Europe, as the historical protagonists of the time (Richelieu, King Charles, Christian IV) act and react based on their "foreknowledge" of the future. We also get to experience the "building up" of the future Army, Navy, and Air Force of the United States of Europe.
In my review of 1634: The Baltic War, I list a number of problems with that volume, and most of them apply here as well. The book spends page upon page dazzling the reader with historical and technical research; little of which serves to progress the story. It is - literally - possible to skip entire chapters of the book without missing anything.
In many ways, 1633 and its sequel are the opposite of 1632 where 1632 focused on the characters and their actions, skimping on the historical background, these volumes focus on historical and technical background at the cost of characterization and pace. This is partly because the story line skips about so much, partly because the characters who worked well in the light-hearted romp of 1632, don't really hold up over 600+ pages. One remains amazed at the idea of a Labour Union-head capable of outmaneuvering the viciously political German princes as well as one of greatest political minds of the time (Richelieu). Who knew Union work was so rewarding...
Despite its weaknesses, 1633 in itself is not a terrible book and if it was a standalone volume, I might even bump it up to 3 stars. Weber and Flint together are not a bad combination, and the story has its moments. The book itself is however let down significantly by the large numbers of the plot lines left unresolved in this volume which basically means that if you buy 1633, you will have to get 1634: The Baltic War as well. 1633 and its sequel are definitely for the the die-hard fan of the series or those who enjoy alternate political history.