Charles Godfrey Leland was a folklorist who befriended magical practitioners and then shared what he had learned about their practices. A classic.
The Golden Bough: A Study of Magic and Religion by James George Frazer (EPUB)
The classic text on mythology, folklore, religion, and magic first published around 1890 in multiple volumes — it’s actually a whole encyclopedia set on the topic. This document is a popular single-volume edition.
Different electronic formats of this ebook are available for download from Project Gutenberg at https://www.gutenberg.org/ebooks/3623