Magic of the Celtic Gods and Goddesses
Carl McColman & Kathryn Hinds
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This is quite a detailed book on the Celtic deities, with a Wiccan slant to it. The authors do not give any actual rituals, however they do provide suggestions for appropriate things you can do to honour them. They also take care to differentiate between known fact and hypothesis based on personal experience. The only criticism may be that though the scope of the book is broad, the depth of information on each individual deity is not that good. This should not dissuade you from reading the book if you are looking for a good introduction to the Celtic pantheon.