I have the Northern Crown books. There are two, a Gazetteer describing the setting and an "Adventures" book, which provides the special rules, like a Players Handbook, for Northern Crown and is not a book of adventures.
The setting, in general terms, is a fantasy North America. However, the English (Albions) do not feature as prominently as they do in actual history. The Dutch control New York and the Hudson River Valley. The Puritans’ Commonwealth is independent. Independent English "cavaliers" control the Carolinas (Carolinga) Etc. The Native Americans have not been conquered, except along the east coast, and not even there entirely. Basically, the Native populations have magic and a unity/foresight they did not enjoy historically. Thus, across the Appalachians, Native cultures remain both viable and dominant.
The Adventure book (ne Northern Crown "Players Handbook") is outstanding. It provides the usual variant base classes and PRCs, almost all of which are very well done, being sufficiently different that they are not "more of the same" and also capturing the feel of a fantasy North America very, very well. The book provides very nice detail of the various cultures, which are presented much like fantasy races, and serve a similar purpose. It is a minor quibble but the Native American cultures thus presented seem a bit "tame," somewhat lacking in flavor that suggests perhaps a desire not to offend but this is a minor quibble as they are still quite interesting on their own terms. There are then the usual sections on equipment, spells, feats, skills etc. Also included are rules specific to Northern Crown - matters of honor, outlook etc. that will make Northern Crown roleplaying a distinctly unique experience. Overall, the Adventures book is a gem. Very well worth the money. If I have a criticism, it would be that the use of the standard base classes from D&D that is suggested often seems forced. Northern Crown would have been better served with more unique base classes. Those presented are almost uniformly well done.
The Gazetteer is what it says it is. It describes the setting and is almost devoid of rules. Given the size of the Gaz, it could have easily formed one book with the Adventures book. In an odd way, it is a good thing it was not, however. The Gazetteer is in no way as good as the Adventures book. To begin, the maps provided in the Gazetteer are ugly, inexact and difficult to read, everything a map should not be. If there are follow up products for Northern Crown, providing better maps should be strongly considered. The Gazetteer entries themselves are scattershot; material is not covered in a comprehensive fashion but here and there, making it necessary to do a great deal of flipping to get a sense of larger areas. The content of individual entries often seems padded to me. The nice details are there but there also seems to be a good deal of extraneous matter that feels "thrown in." The overall feel of the Gaz is "loose" or "vague." The Gaz is neither tightly written nor designed. While this may appeal to those looking more for "suggestions" that do not "restrain" a game, those looking for a firm sense of Northern Crown as a setting will actually find more of this in the Adventures book than in the Gaz itself. On its own merits, the Gaz is unimpressive and not worth the money asked. However, it should be considered with the Adventures book.
Taken together, the two books of Northern Crown form a very compelling and, IMO, outstanding setting, well worth the purchase. Northern Crown is definitely something new, not seen before. The rules execution is inviting and very well done. While the Gaz is much less well done, the Adventures book is so good and has so much potential for great gaming that the Gaz, merely by fleshing out some greater details, warrants a buy decision. If you can only afford the Adventures book, however, buy it, even without the Gaz. You will likely not be disappointed and will have a number of new vistas open to you. You really do not need the Gaz and can always purchase it latter once you catch the Northern Crown bug.
Northern Crown is, then IMO, a definite setting to be had, whether to play on its own terms or to steal from for a home brew. It is full of great ideas. I’d give it 4.5 out of 5 stars. Its got a definite beat and you can dance to it.