Shineylight,
Anything by H.P. Lovecraft or August Derleth will be awesome! August took H.P.'s mythos to a more traditional horror direction so they are more accessable than H.P.'s work. Clive Barker's "Hellbound heart" is a great story that Hellraiser was based on. "Imajica" is another great story by Clive that isn't really horror but very awesome and long. "Illuminatus" by Robert Anton Wilson and after that you could check out, "Shroedinger's Cat"; they are both trilogies. That should keep you busy for a few months.
Rhizo
Anything by H.P. Lovecraft or August Derleth will be awesome! August took H.P.'s mythos to a more traditional horror direction so they are more accessable than H.P.'s work. Clive Barker's "Hellbound heart" is a great story that Hellraiser was based on. "Imajica" is another great story by Clive that isn't really horror but very awesome and long. "Illuminatus" by Robert Anton Wilson and after that you could check out, "Shroedinger's Cat"; they are both trilogies. That should keep you busy for a few months.
Rhizo