The most interesting thing about the Celts, from a fantasy worldbuilding perspective, is how uninteresting they are. I don't mean this as a slight against Celtic culture or anything, but just an observation. Dungeons and Dragons was a stew made of many influences, but almost all of them run through Celtic culture at one point or another. For example, I learned that Conan the Barbarian drew inspiration from Celtic myth, which actually didn't surprise me in the slightest once I thought about it for one second. And The Celts Campaign Sourcebook in particular often cites Caesar's The Gallic Wars, and maybe it's unfair to the Celts to have their culture described by their worst enemy, but it does go to show the influence they had on classical culture, even if only as antagonistic outsiders.
So many of their mythological themes are embedded bone-deep in English-language fantasy, undoubtedly due to the fact that they are an unalienable part of British culture. It is probably impossible to even talk in English about magic and monsters and heroes without incorporating some level of Celtic bias. Even Tolkien, who self-consciously rejected Celtic inspiration, wound up using a few characteristic tropes in building his world.
It all lends the "historical fantasy" of the Celts Campaign Sourcebook an eerily familiar feel. I implied it was "uninteresting" in the first paragraph, but a better description would be "unsurprising" (and no, I am no going to go back and edit my opening). The Celtic-inspired fantasy of this book feels like what fantasy is supposed to feel like. And much like with the Vikings book, it falls short only in the sense that you could build a core book and a line of a hundred supplements off this book's pitch and wind up creating a very satisfying fantasy world.
It was kind of a surreal experience, seeing a version of D&D that evoked Exalted, with its over-the-top "heroic feats" (like jumping a chariot or throwing a spear with your feet) or Changeling, with its talk of faerie worlds just out of sight, where time flows differently and from which men may never return. Mainline AD&D never quite got there, and it probably couldn't handle it if it did, but it's kind of fascinating to think of a version of the game where Neil Gaiman made it into Appendix N.
The Celts Campaign Sourcebook wasn't quite that, unfortunately. Probably because it wasn't interested in building a coherent world. The Celts were a wide-ranging people, and the book acknowledges that. As a result, it's mostly a grab-bag of Celtic-related topics. Useful, inspirational, but it never congeals as a setting.
Strangely enough, a big culprit is the rules. I wouldn't have thought that possible, given that the Druid and the Bard are two full character classes that draw a line of descent almost directly back to Celtic culture, but they get converted in some baffling ways. Druids lose their weird organization, but also their high-level granted abilities and a huge chunk of their spellcasting. Bards lose their thief skills in exchange for basically nothing (well, the ability to use Bestow Curse, but they could already do that with their spellcasting, though admittedly not until level 10, raising a ton of balance questions).
I think what's going on is a misguided fidelity to an ideal that never existed. The two most Celtic classes get nerfed in the Celts book because their abilities don't map neatly to surviving Celtic stories, but there's no real exploration of the new stories you're supposed to tell. The Celts didn't have roleplaying games, but if they did, you can bet your ass that they'd see no contradiction between "high fantasy" and "historical fantasy," and certainly wouldn't put up with weaksauce "realistic" magic, just because a particular feat hadn't been seen yet.
Chariot-Jumping requires 4 non-weapon proficiency slots! Over and above the one you spent to get chariot driving in the first place. That's way too much to charge for a niche ability whose only real use is to look cool. It's frustrating to see something that's so close to the point of heroic fantasy and yet so far from actually getting it. But then, that split - fantastic things are caused by explicit magic, anything else has to cleave to a fairly rigid implementation of the laws of physics - is at the heart of what Dungeons and Dragons has always been. It's something that has occasionally hurt the game (as it does here), but I can't say it comes as a surprise.
Overall, the Celts Campaign Sourcebook is probably the weakest of the historical reference series. I mean, I still have two to go, so this is a verdict that might be overturned, but despite having some genuinely good information, the laws, customs, and economy of the Celts are so deeply ingrained in what we think of as "generic European fantasy," that this book sometimes feels like generic European fantasy, salvaged only by the more non-AD&D-friendly bits of Celtic cultural weirdness. I'd still recommend it, because it states a lot of things that most fantasy setting guides take for granted, but it doesn't feel nearly as subversive as Charlemagne's Paladins or Crusades (though it may be that I'm simply getting jaded after reading so many good "Medieval Europe" supplements, and Celts merely has the misfortune of being the most recent).
Ukss Contribution: Avalon, the deathless island where all wounds are healed. I like this one because it somehow manages to be both sacred and spooky. There's probably a reason why it gets reused so often.