I can strongely recommend you to read this book: The World of the Gauls, Foundation(s) of a Celtic Philosophy by Antón Bousquet. No fluffy, flowery “how to” book, but a very profound examination of the worldview and the Gods of ancient Gaul. The basis is formed by the three realms of Dubnos, Bitu and Albios. Check out the table of contents on Amazon. In my humble opinion one of the best books on Celtic Polytheism.