So I am occasionally asked for good places to start studying Celtic culture or otherwise learn about the Celts. This would seem like a pretty straightforward request but actually its a really complicated topic; what I usually suggest is that people start with a specific culture rather than going to the wider, older idea of 'Celtic'. What many people mean when they say Celtic is a single homogenous or unified culture which they think is a monolith and therefore easy to study. However Celtic is actually a loose term for a constellation of cultures related by language, art, and mythic motifs which were at one point influenced by or grew out of a single source culture - that source culture is understood via archaeology and commentary from outside sources like the Greeks and Romans. Even that older source culture however wasn't actually a cohesive group but an array of groups that shared a root language. Historically there were a multitude of cultures labelled Celtic; currently there are 6 that are referred to as Celtic nations: Ireland, Scotland, Wales, Cornwall, the Isle of Man, and Brittany. So studying this becomes a bit difficult.
Its important to note several things here as well:
1. This is a field which is constantly changing and in which theories come and go. Scholarship is fluid not fixed, for the most part, and there are several perpetual fierce debates about things. A theory that may have been widespread 50 years ago may still be seen as valid or may have been entirely refuted - or may be argued over today. Bias is a factor that must be kept in mind in this field and new information must be incorporated regularly.
2. Despite the way it is often discussed there is no 'Celtic pantheon' per se. The group of deities labelled as Celtic represent an array of beings from across the various Celtic cultures: only a few are pan-Celtic (ie found across all or most of the groups) and most of the cross cultural ones weren't historically worshipped together in the way that the idea of a Celtic pantheon. Its also vital to note that the handful of pan-Celtic deities were not cohesive between cultures - the Irish Lugh isn't identical to the Welsh Llew even if they likely share a common root and are seen as cognates.
3. Celtic doesn't equal Irish. I often see people treating Celtic and Irish interchangeably and that is inaccurate. Ireland is one culture that falls under that Celtic language speaking cultural umbrella but not the only one.
4. There is a huge amount of romanticism around this subject, some of it harmless and some of it manipulated for nationalist, fascist, and supremacist ends. When studying this subject and especially when vetting sources extra discernment and caution is needed in my opinion because of this, particularly when considering sources outside academia.
That all said I will offer a few recommendations here, with the caveat that these are based on my opinion of good places to start with the subject and my own interest in this is tangential.
- Rees and Rees - Celtic Heritage. A bit dated at this point, but a solid resource.
- Simon James - Exploring the World of the Celts. this isn't what I'd call an academic text but is a great intro to the subject and includes a large number of illustrations and photos.
- Koch and Carey - The Celtic Heroic Age. Focusing on literary sources across Europe, Ireland, and Wales this is a good book to get an idea of many of the sources people pull from.
- Barry Cunliffe - The Ancient Celts. A wide overview of the subject. I would also suggest Cunliffe's work on 'Celtic from the West' with the understanding that is a debated theory.
- Sjoestedt - Celtic Gods and Heroes - definitely dated (originally published in 1949) but an easy accessible introduction to the subject and it covers all the highlights. Definitely don't rely on this one alone.
- MacKillop - A Dictionary of Celtic Mythology. Caveat on this one, MacKillop doesn't clearly cite his sources in the text and some of his material can't be verified outside his own book. So take this one with a grain of salt but it can be a good starting point and a good quick reference.
- Miranda Green - assorted titles. Green has written extensively about various aspects of Celtic culture. As with MacKillop she requires a caveat in that she is primarily an archaeologist and her material focused on that is strong but when she theorizes outside of that always double check her assertions which can be creative.