I haven't read the book from cover to cover yet, so I freely admit, it is possible that I have missed a detail or two. Now, I do know a thing or two about this era and accordingly, I had certain expectations concerning how the setting could be implemented. Based on these expectations, my impression so far were rather disappointing. Or to summarize: I expected a much better book. Dark Albion is pretty mediocre. At best.
The Dark Albion setting is pretty good whenever it stays true to the historical foundations. Mostly because the 15th century is a great place to run adventures in. But: Whenever the setting deviates from these historical baselines, the quality drops rapidly. The fantastic elements usually jar with the rest of the setting most of the time, and the author doesn't have a good feeling for the cultural mores of the era. For instance, essential stuff like the massive authority loss of the church in the post plague years that pretty much define the era, are ignored.
A huge part of this decline in quality is the need to dilute many of the actually interesting conflicts is by forcing them into an inherently childish black and white, good vs. evil pattern. This enforced immaturity diminishes these events, and nowhere is this as obvious as with the 100 years war itself. Sucking the ambivalence and complexity out of it to make place for stupid, inherently evil frogmen is pissing away what makes the setting good by trivializing it.
There is quite a lot of stuff that didn't made it into the book, and these omissions are bizarre. There is quite some stuff that would clearly belong in this setting (i.e. Jeann d'Arc, John Wycliff, the Great Schism and Portugal, not to mention that any setting in that era that does not exploit Gilles de Rais is weak. Weak.) that, as far as I can tell do not occur in this book.
The most 'problematic' of these in a 'I wanna be offended' definition of the word is the complete erasure of European Jews. No Rabbi Loew and his Golem for you, I guess. Or kabalistic magic. Just to get this completely clear: This isn't some hidden or indirect accusation of antisemitism. It is just genuinely awful writing. The excluded stuff is rife with adventure potential.
For instance, no Portugal means no Henrique the Navigator, and consequently, no exploration of the Atlantic or the African coast. Also no Jews means no PCs saving the innocent people of that nice ghetto from the angry mob's pogrom (also no belle juives if you want to spice your 15th century mayhem with a bit of 19th century romanticism). These are adventures that basically write themselves and not including the basics is just wasting the setting's potential. Again.
(However, the most stupid part is the replacement of the Kalmar Union with a bunch of anachronistic Vikings. A barely held together alliance of kingdoms, dozens of nobles almost constantly on the brink of unrest, etc - or basically yet another campaign writing itself - is replaced with… pointless, boring Vikings that do not fit into the era at all. You could just as well include Napoleon Bonaparte in the mix and it would still be less anachronistic in a mid 15th century setting than fucking Viking raiders.
Now, the rules part is, as far as I can tell, pretty good. I especially liked the stuff aboutt demonology; I think that this is a genuinely good expansion of simple D&D-ish rules. If I used those rules, I would very likely use these demons. Even for the Runequst-based rules I use for a game like this, there are some genuinely good ideas I plan to implement in my games in some way.
The random event tables were neat as well. The rest of the rule-stuff wasn't as great, but seems still pretty solid to me. So, the actual mechanical part of the game are by far the best part of the book, but I'm not sure that this is worth the money. The setting part definitely isn't.
Oh, and the artwork is nice, but most of it seems to be public domain stuff anyway. You get pretty much the same effect from looking at some of the works by Dürer, Cranach, van Eyck, Bosch and Breugel, plus the book of hours of Jean sans Peur. The maps are nice too, I guess, but - it is late medieval Europe. It is not like maps of that place are particularly rare or hard to find.
If you want to have a 15th century historical fantasy setting, only you know, in good, take a look at Andrej Sapkowski's Hussite Trilogy (Narrenturm, Warriors of God, Lux Perpetuam), which is vastly superior in every conceivable way. Mostly because Sapkowski actually understands the era he writes about (any claims that these books are well researched would actually hold any water) and that world building in a historical context should be based on expanding and not replacing the historical foundations with the fictional elements. Also, Sapkowski writes for an actually mature audience, so he doesn't try to force the historical events into a inherently juvenile clear-cut good vs. evil pattern. Or hides behind blatantly transparent name changes when he writes about the Roman- Catholic Church (and how it was fucked up enough at the time to eventually spawn the Reformation).
Also, Sapkowski, actually being Polish doesn't swoon over Polish historical figures that much. Naïve Polish fanboyism wasn't exactly what I expected to find in Dark Albion, but there it is.
All in all, it is a pretty meh book. There are some good ideas in it, but most of those, you can also find in an afternoon of wikipedia binging or the average library. The demonolgy stuff is pretty much the highlight of the book, but it is pretty much the only real highlight. So yeah. I expected something a lot better.