If you're interested in some of the life and afterlife of Celtic myths, I'd wholeheartedly suggest The Celtic Myths that Shape the Way we Think by Mark Williams. I finished it a few days ago, and it was really good. Went into detail about what we can actually know about Celtic myth (hint: pretty much nothing), what even 'Celtic' means, etc., but also talked about some of the most famous stories, what our sources are, and how they've been adapted since. It was quite an interesting read.