Fairy Faith in Celtic Countries by W Evans-Wentz
This is a testament in itself, as this was first published around 1890. A collection of reports, and stories of faery encounters in every Celtic nation, ranging from firsthand accounts to "a friend of a friend" legends to stories handed down through the generations. After a presentation of mass information on the modern fairy faith, it goes on to relate the ancient fairy beliefs held by the Celts of old, as recorded in their mythology. The Fairy Faith in Celtic Countries is a source document nonpareil and an absolute must read for anyone with a serious interest in the Fairy Faith.