Why are we afraid of broken mirrors? Ancient taboos, fears about drowning, doppelgänger, demons, a tool of divination, the soul's prison? Trace the superstition back to myths of Narcissus, Echo, Hindu death spirits and the Slavic gypsy Fate goddess. Dig deep into the collective psyche. You may find a better history of a superstition in a comprehensive encyclopedia of mythology or even religion.
The Woman's Encyclopedia of Myths and Secrets is a first-stop reference center. The author has researched the topic and provides a succinct summary tracing it across all cultures/all religions/since recorded history. She covers myths, gods, goddesses, symbols, folklore, ancient customs, Christianity, random abstract concepts, pagan topics, Wiccan topics, etc.
In The Woman's Dictionary of Symbols and Sacred Objects, Barbara G. Walker illustrates and arranges symbols thematically and provides the history and mythology of each.
Encyclopedia of Signs, Omens, and Superstitions: Ronald Donald Papon took on the late Victorian astrologer's name. Don’t let the odd name fool you: impressively broad, including myths, rituals, legends, customs, holidays, place names and origins.
The Dictionary of Symbols includes these categories: psychology, language, literature, mythology and Jungian dream symbols.
The Secret Language of Symbols: a Visual Key to Symbols and Their Meanings is both a useful reference book and a beautiful art book. A Jungian theme of investing symbols with power is examined across art, psychology, nature, mythology, religion, and the occult. The book first impresses you with the power of symbols, how they are used then guides you through objects, mundane to fantastic.
Encyclopædia of Superstitions brings folklore and faeries to life from the British novelist who also wrote classic reference books.
Encyclopedia of Religion: This 15-volume reference set is a go-to source for concise articles on all religions, mythology, folklore, origin stories, philosophy, psychology, the occult, literature, theater, etc. The eBook edition of The Encyclopedia of Religion, available through Gale Virtual Reference Library, is the second edition of a resource that is considered a standard reference in the field. It presents a cross-cultural approach that emphasizes religion's role within everyday life.
You can find more resources in my Sleuthing Slippery Superstitions, Symbols and Mythology booklist.