I don’t use a lot of online resources, since those tend to be dodgy in the extreme haha. But I can recommend some books! You may be able to get these through the library, or a university library network if you have access to that. Some of them are easily found on Amazon and Ebay too.

Some sites:

So Brythonic gets a little wishy-washy in terms of definition because sometimes you are talking about Welsh mythology and sometimes you are talking about Iron Age Brittonic religion. Linguistically, though, Welsh developed out of Gallo-Brittonic, something that’s really obvious when you look at names changing over centuries. When most people talk about Brythonic religion they’re talking about Welsh mythology and looking back to Brittonic Iron Age religion as a filter to figure out what religion would have been like in earlier time periods than the Mabinogion dates to. I recommend this site and this site on the Mabinogion because it gives nice historical analyses of what political stuff was going on at the time to inspire the stories, that helps in sorting out what’s what.

Tacitus wrote about the Britons in Agricola, and I find it’s generally always best to go to those sources and read them, because a lot of books will talk about these quotes assuming that you know the original context. Lots of information. Caesar’s account when he briefly tries to invade Britain (Chapters 12-14 unless tactics are your thing in which case keep reading) in the Gallic War gives a little bit of information too, but as always, take with a grain of salt. Something to keep in mind with Roman writers is that they were always pushing some kind of propaganda for a war machine when writing about their enemies, and most Roman writers actually did not visit the places they wrote about and were relying on hearsay. So take everything with a healthy heaping of salt. For example, all that stuff talking about how the Britons had tattoos? All of it just quoting a single line from Caesar, which may equally be translated as “All the Britons decorate themselves in blue glass” which was, in fact, an archaeologically attested fashion, since blue bangles and blue enamel was popular in Britain. And nowhere does he actually mention woad! It’s something added into a lot of translations but isn’t there in the actua Latin. So yeah, use your own judgment, proceed with caution. There are definitely other writers talking about Britain, but I don’t know them as well unfortunately. :/

Finally, this lovely webpage has a dropdown which will let you check if a god or goddess is attested in Britain or not. This is a big help in sorting out if someone is relying on Pan-Celticism or not when they say a god was worshiped in a specific pantheon. I find Wikipedia is not terrible at talking about deities, so long as you also check out the sources at the bottom. The best site for this no longer exists sadly. :c

And some books:

Pagan Celtic Britain by Anne Ross is the de facto Brythonic Celtic religion book, as far as I know. I hear only good things about it, though I’m still working on reading through it. (Gaul is my focus so I’m more help there than here haha.)

The Ancient Celts by Barry Cunliffe and The Celts by Simon James are great general overview books, just be wary of Pan-Celticisim. That’s the notion that all Celtic cultures are the same, that there’s just one Celtic culture and one Celtic religion, and that isn’t true. A lot of myths about the ancient Britons start because someone took something from Gaul and said it applied to Britain. (Examples of that I can think of: 1. They fought naked! [Nope, a couple Gauls at the Battle of Telamon took off some clothing so they didn’t have to fight the underbrush and it got turned into semi-mythology by the Romans, most Britons and Gauls for that matter would have worn clothing or chainmail into battle], 2. The Britons had no kings and elected their leaders! [No, this is true of Gaul but not of Britain, in Britain they still had kings and queens over their tribes].) Anyway, Barry Cunliffe has written a lot about Iron Age Britain so if you can find more of his academic books, those are a safe bet. He does have some questionable theories though, and some of his books haven’t been updated since the ‘70′s, so it strays into the occasional Pan-Celticism.

War, Women and Druids by Philip Freeman is a nice primary source book of quotes by various contemporaries of the Gauls and Britons talking about them. Just be wary that he does not specify if he’s talking about Gauls or Britons, so pay attention to the blurbs in between the quotes to see who the quote refers to.

And the elephant in the room hahaha, Miranda Green. Miranda Green is a prominent writer about Iron Age Celtic religions and about Britain in particular. She’s a good writer, and her books make excellent reference, especially when looking for photo references of things. However, her analysis is... famously not very good scholarship. She jumps to conclusions and comes up with conclusions that aren’t actually attested in the evidence. Her books make great references, but be wary of any statements she makes about their meaning or about the culture. I think my favorite book of hers is A Dictionary of Celtic Mythology which is basically just purely reference. I recommend her books highly as reference and just because they’re well-written, but not if you’re just starting out, because they require a bit of legwork to sort out what’s fact and what’s a theory!

But yeah my focus is Gaul, so I will be much more help there!! I hope this is helpful. A really helpful thing too will be to go to the sources section on Wikipedia pages that interest you and click through or try to find the sources in whatever libraries you have access to. It’s a lot more accurate than just reading the Wikipedia page.

And a caveat: It has been such a long time since I kept up with this stuff. I’m a few years out of date. So there may be many great resources that I’m not currently aware of out there.