Listmania!
Trans-emperical Reality
Passport to Magonia: On UFOs, Folklore and Parallel WorldsPassport to Magonia: On UFOs, Folklore and Parallel Worlds by Jacques Vallee
Buy used from: £50.56
A good starting place to the notion of trans-empericalism
Fairy-Faith in Celtic CountriesFairy-Faith in Celtic Countries by W.Y. Evans-Wentz
Buy new: £12.74 / Used from: £10.32
Evens-Wentz looks at the Celtic Fairy-faith and comes to similar conclusions as the other authors here
Testing the SpiritsTesting the Spirits by Elizabeth L. Hillstrom
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A Christian and scientific analysis of a bunch of trans-emperical phenomena, with some interesting discoveries.
Principalities and powers; the world of the occultPrincipalities and powers; the world of the occult by John Warwick Montgomery
Buy used from: £8.25
Montgomery takes a fresh and open-minded Christian look at a wide-range of trans-emperical phenomena, although provides no general synthesis. A good starting point.
Daimonic Reality: A Field Guide to the OtherworldDaimonic Reality: A Field Guide to the Otherworld by Patrick Harpur
Buy used from: £22.27
Haven't read this yet, but I suspect this to elaborate on what the other authors listed here have discovered.