The Call of Cthulhu by H. P. Lovecraft, the story that started it all
The Cthulhu Mythos is based on the writings of H. P. Lovecraft, and central to it are the Elder Gods such as Cthulhu. Its unholiest text is the Necronomicon by Abdul Alhazred. The Cult of Cthulhu worships Cthulhu, following the teachings of the Cthulhu Mythos. Although it is deceptively called a "mythos", it is nevertheless entirely true. Here is a video based on an audiobook of The Call of Cthulhu by H. P. Lovecraft:
