Rosemary Guiley, The encyclopedia of Magic and Alchemy (Infobase, 2006; ISBN0-8160-6048-7), p. 38
Benjamin Thorpe, Northern Mythology: comprising the principal popular traditions and superstitions of Scandinavia, north Germany, and the Netherlands (London: Edward Lumley, 1851) v. 2 pp. 186 et. seq.
Stokker, above; see generally the chapter, "The Black Book"