So, there's a Unaussprechlichen Kulten wikipedia page.
Unaussprechlichen Kulten - Wikipedia, the free encyclopedia
"Unaussprechlichen Kulten (also known as Nameless Cults or the Black Book) is a fictional work of arcane literature in the Cthulhu Mythos. The book first appeared in Robert E. Howard's short stories "The Children of the Night" (1931) and "The Black Stone" (1931) as Nameless Cults. Like the Necronomicon, it was later mentioned in several stories by H. P. Lovecraft."
"The following is a fictional account of the origin of Unaussprechliche Kulte and its significance in the mythos.
Unaussprechliche Kulte is believed to have been written by Friedrich Wilhelm von Junzt. The first edition of the German text appeared in 1839 in Düsseldorf. The English edition was issued by Bridewall in London in 1845, but contained numerous misprints and was badly translated. A heavily expurgated edition was later issued in New York by Golden Goblin Press in 1909. Original editions in German have a heavy leather cover and iron hasps. Few copies of the earliest edition still exist because most were burnt by their owners when word of von Junzt's gruesome demise became common knowledge. An edition is known to be kept in a locked vault at the Miskatonic University library.
The text contains information on cults that worship pre-human deities such as Ghatanothoa and includes hieroglyphs relating to the latter. There is also information on more recent cults including that of Bran, The Dark Man. It is from this work that the tale of the doomed heretic T'yog is most commonly sourced. The principal obscurity of the book is von Juntz's use of the word keys—"a phrase used many times by him, in various relations"—in connection with certain items and locations, such as the Black Stone and the Temple of the Toad (possibly associated with Tsathoggua) in Honduras."