The "Unaussprechlichen Kulten" is a fictitious book invented by Robert E. Howard while adding to the Cthulhu mythos. Like its better-known cousin, the Necronomicon, it is supposed to contain dark secrets about ancient gods and stuff.
He first introduced it in his story "The Children of the Night", where it was called "Nameless Cults". But since it was "written" by a German called von Junzt, it needed a German title. Lovecraft wanted to call it "Ungenennte Heidenthume", but August Derleth's suggestion, "Unaussprechlichen Kulten", was the one that took hold.
Unfortunately, Derleth screwed up quite a bit.
He picked the wrong case: the accusative instead of the nominative. The title really should be "Unaussprechliche Kulte", without the "n"s at the end of the words. (Perhaps it's the fault of this book title, or perhaps there are other sources, but I've noticed similar errors from many people trying to write something German-sounding. There probably is the idea floating around in some people's heads that all German words end with "en".)
The second little problem is that the book's title sounds ridiculous in German. What Derleth meant to express was "Unspeakable Cults", because "unspeakable" or "unutterable" was one of the Lovecraftians' favourite expressions. But "Unaussprechliche Kulte" rather translates to "Unpronounceable Cults". Just cults with names you can't pronounce properly. After all, Derleth did have problems with foreign languages.
Sources:
http://www.hplovecraft.com/creation/tomes.htm
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