As the first book in AD&D's historical reference series, Vikings Campaign Sourcebook has not yet worked out the "Historical/Legendary/Fantasy" split that we see in later volumes. It is, instead, just a pure Viking fantasy game. It's set in the historical North Sea region, but there are giants and Linnorms and Dock-Alfar, and all the other elements of Norse fantasy, maybe toned down a little, but still explicitly magical and presented without much by way of explanation.
It's actually a pretty great setting that could easily have supported its own entire game line, and if this book has a flaw, it's that it's ultimately pretty shallow about something that has a lot more depth to be mined. For example, it never mentions that the seiðr was traditionally considered a feminine practice. We get as complete a list of Odin's nicknames as I've ever seen, but nothing about his gender-bending.
Granted, it's 1991, and whoa, that would be forward thinking even for a game not as characteristically hidebound as AD&D, but on two separate occasions the book stops to point out that Viking women had an unusual amount of autonomy for the middle ages, and thus it was appropriate to make female Viking PCs . . . and maybe they could have tossed in a bone about how magic was basically considered a women's job. Every adventuring party needs a mage, and thus even the most "realism" focused D&D groups should have had no problem with at least one female character.
I mean, it's sort of brought up. "The sagas make mention of several wizardesses, some favorably." But even setting aside the awkwardness of the word "wizardesses," the book got magic completely backwards.
Which is strange to me, because it has an extensive-looking bibliography. Is this a case of historiography changing dramatically in the last 20 years? Were the books cited here guilty of projecting contemporary sexist assumptions back onto the Viking culture and it's only because my own introduction was from a new generation of scholarship that I even noticed? Possible. Maybe even likely. At least, it's something to look out for.
Although, if the fantasy elements have been done better in other products, the Vikings Campaign Sourcebook does manage to do the thing that these green books have excelled at and make us really stop and think about what life is like in a Medieval European-inspired fantasy world. It talks about architecture, diet, and fashion, as well as laws and customs, and it gives a great sense of setting. It's not the best of the series, but it's easily ahead of 90% of the pure fantasy stuff out there.
It's also the strongest, mechanically, of any of the green books I've read. Which is weird, for being the first, but I think comes down to never trying to pretend to be purely historical. The thing about allowing specialist wizards, but not mages is still a little weird, because spell schools aren't narrowly tailored enough to make those kind of blanket bans very thematic, but we are at least spared the spectacle of clerics in an ostensible "non-magic" campaign.
Plus the new classes are pretty cool. The Berserker is basically the AD&D barbarian done right, and it's fun that they get wolf and bear shapes at higher levels. "Mostly fighters, but with a few magical tricks" is a class niche that AD&D should have gone to more often.
And the Runecaster class, despite the gender mixup, is pretty decent as well. Its powers are generally weaker than mages or priests (with the caveat that they're not leveled, so it's possible to get some unusually strong abilities at level 1), but it fits better in a historical milieu than any of the "historical" rules modifications we've seen so far.
Overall, this was a pretty straightforward book. Vikings and Norse mythology were already pretty deep in D&D's DNA, and so the setting and the system mesh unusually well, better even than many pure fantasy setting explicitly designed for D&D. The result is a book whose monsters and magic felt a lot like what D&D should have been doing all along, but also one that is not really surprising in any way. Let this be a lesson to would-be game designers, specificity is good.
Ukss Contribution: . . . he said, while preparing to come up with a new element of the most hopelessly muddled fantasy world ever devised. Anyway, Vikings ate kale. I don't know why this is such a surprise to me. I guess because I first heard of kale about five years ago when it started becoming a fad, and I just assumed that it was a new thing. You know, like those new crops they're constantly discovering. . .
Yeah, I could have preserved my mystique as a cool customer by keeping this bit of surprise to myself, but it did amuse me to think of these macho Viking guys eating like a California vegan, and so I will memorialize the smile this put on my face by adding extensive kale production to Ukss.