The Blessed Isles

GURPS Celtic Myth: This book contains some information on the celts. Including their marriage patterns, how they play, and some of their world. It also contains a brief telling of the Celts and their world.   AD&D HR 3: The Celts: this book is available on Drivethru as a pdf. It contains different information than GURPS Celtic Myth, not including the mythology that the former book contains.   The Ancient Celts, Second Edition: this book by Barry Cunliffe goes through the celts and their culture. From archaeology to anthropology, everything is there to be had.   The Mabinogion: This book provides the mythic cycle of the Welsh people.  It contains tales about King Arthur, his sons, and his knights.