Fictional books are always fun for me, and one of the things I love about Cthulhu mythos stories- the Necronomicon is well-known, but also Unaussprechlichen Kul

Fictional books are always fun for me, and one of the things I love about Cthulhu mythos stories- the Necronomicon is well-known, but also Unaussprechlichen Kulten and De Vermis Mysteriis. In a similar, but earlier vein is The King in Yellow from Robert W. Chambers.