First a couple for the kids or parents with kids:
The Ancient Celtic Festivals: and How We Celebrate Them Today by Clare Walker Leslie
A Child’s Eye View of Irish Paganism by Blackbird O’Connell
Then some more adult books:
Teagasca: The Instructions of Cormac Mac Airt by C. Lee Vermeers
The Secret Commonwealth and the Fairy Belief Complex by Brian Walsh
Stalking the Goddess by Mark Carter