Apropos of nothing, really, I want to know more about the Picts, i.e., their culture, language, etc. From my understanding, we don't have a ton of knowledge about them, but after some cursory Google searches, I'm not sure what books would be good to start with for someone who is mostly unfamiliar Celtic cultures. I'd really appreciate it if anyone could point me in the right direction!
I'd normally refer you to the AH booklist, but there seems to be a bit of a gap there where I'd normally hope to find recommendations.
Others will have their own suggestions, but for me the gold standard in terms of a comprehensive and up-to-date survey resource is Alex Woolf's contribution to the multi-volume New Edinburgh History of Scotland, From Pictland to Alba (2007). It's worth pointing out that you are not going to get a straightforward narration of anything from this book – the evidence we have is simply too scanty for any such account to be possible and so Woolf, and anyone else writing about the Picts, spends a lot of time debating possibilities. Alfred P. Smyth's Warlords and Holy Men: Scotland 80-1000 (1989) offers a much clearer narrative, but it's also rather out of date and Smyth is, let's say, a more controversial historian than Woolf is, though the people who have negative views of him do so because of his work on the Anglo-Saxons more than that on Scotland.
Tim Clarkson, who in my view is a solid historian but who unlike Woolf has no university affiliation, has written widely on northern history in this period and his volume The Picts: a History (2012) is also worth a read. You can use the bibliographies that both books possess to explore further once you have solidified your grounding in this area – it's fascinating but, frankly, very tricky and frustrating, and to enjoy it you really need to enjoy thinking of history as a form of detection, and be interested in both contemporary language and art.