This fairly short work is another of the various bizarre releases of its era, which has little back story and content which fluctuates wildly from subgenre to subgenre. Supposedly based off an older German version (said here to be popular, but which may indeed not exist at all), it first lays out some philosophical principles tied with the use of willpower and psychic force, then...
Charles Leland (who is notable for his most famous work, Aradia, and for some other significant folkloric work especially in Italy) wrote this work under a different title originally but ended up creating this variant (which was published posthumously) for an American audience. It is quite good, and fuses a bit of fairies and folklore in with its essentially psychological content...