Hardcore fantasy readers might find The Secret Commonwealth of Elves, Fauns and Fairies by Robert Kirk and Andrew Lang to be interesting reading. Lang, a nineteenth-century folklorist, had printed and wrote a long introduction to a seventeenth-century manuscript by Kirk.
Both parts are worth reading if you like the topic. The language is old, and by our standards the spelling is eccentric, but you will see where this little book has had an influence on contemporary fantasy. Definitely read the footnotes and make free use of Google.