I enjoyed it fairly well, the characters are likeable, and it has interesting subject matter, but the big downside is that this is half a novel in terms of plot development and there's a bit of been-there done-that in terms of story since the basic driving force is the attempts by envoys of the United States of Europe to get the emperor of China to give them trading status, which is very much like the basic plot of 1636: The Mughal Mission. I would actually say it's more of a 7/10 in terms of rating but I couldn't bring myself to 4/5 so I gave it 3 stars. I will say that I felt like this was a dramatic improvement over Mr. Cooper's previous Ring of Fire book, 1636: Seas of Fortune, which had a lot of rushed developments and WAY too many characters, not to mention that it was actually two novellas that only vaguely had anything to do with each other (they were both set in the western hemisphere and both ended ~1636) spliced together. It's notable that this book is not a direct sequel to that, though it does build somewhat on one of the events of that book, the Japanese conquest of Manila, which I thought that Iver Cooper did a good job of explaining here in terms that made it seem much more reasonable a development.
As for story, in the 1630's China was in a very weird place. Although it was still one of the world's largest, most prosperous, and most stable nations on the outside, on the inside it was fraught with problems (increasingly ponderous bureaucracy, imposed cultural/technological inertia, power struggles between elements of the Imperial court, perennial droughts leading to widespread famine, inability to pay the army regularly leading to widespread desertion and the growth of bandit armies into serious threats to the nation, and the growing unity and military sophistication of the Jurchen of Manchuria) that were about to spin out of control leading to the collapse of the Han Ming Dynasty and the rise of the Manchu Qing Dynasty. Despite the fact that the up-timers know just how badly China is about to come apart at the seems and the fact that they may be too late to do much about it (China's crop failures were mostly the result of the global cooling trend at that time (the Little Ice Age), for instance, not something that foreknowledge can change much (outside of providing impetus for external trade)), they also know that China is the pretty much the only place where large quantities of certain necessary industrial materials (like Tungsten) can be obtained in quantity and where there's the infrastructure in place to make mining possible (without a major colonial effort), so to China the up-timers must go! However, just because you WANT to trade with 17th century China and have knowledge and technology they could use, doesn't mean that 17th century China will treat you like anything other than uncivilized barbarians, and this is the core of the story, figuring out a way to be taken seriously and given an audience with the Emperor while also gauging whether it would be possible to actually obtain the resources that the USE needs and whether there's any chance of keeping the Ming Dynasty from imploding. Fortunately for the small team of up-timers and attached Dutch/German diplomats, merchants, artists, scientists, they make an ally in the form of Admiral Zheng, a man who is that rarest of 17th century Chinese elites, one who realizes that the world doesn't end at China's borders. With Zheng's support, the up-timers are at least given a chance to make an impression, and that's pretty much the book: they go to China, they make friends, they exhibit their technology and knowledge, and when more desperate events occur, they pitch in. As I said before, it's fairly interesting, but frankly that's as far as it goes, this isn't exactly a gripping page turner, and it's not likely to appeal to you unless you're a pretty hard-core Ring of Fire nut (like me). That said, I will be interested if it gets a sequel, as I felt this book laid the groundwork for a much bigger and more interesting story, but it just never quite gets there. So we'll see. I will also say that i thought Cooper and Flint did a good job with their depiction of 17th century China, which is a very strange place from a Western perspective but not necessarily a bad place, just very alien and very hidebound.