1. ’66/’67: Story involving a boy who wakes up to witness small, lizard-like creatures entering his body and carrying away bits of it. Of course no one believes him, and this keeps happening. He discovers the creatures are somehow taking the place of the bits they carry off. In the end, he meets a copy of himself, assembled from the bits that had been stolen, and when this copy announces itself as the real boy, our protagonist disintegrates into a swarm of the lizards. End of tale.
2. My mom had a paperback book that I read a few times when I was a kid that I thought was called Black Mass or Black Sabbath. It was a black cover with a pentagram embossed on it and (I think I’m remembering correctly) a satanic goat head. The story was about a girl, deja-vu with a tree (because she was reincarnated), and a man she falls in love with who ends up being a satanic priest out to kill her (again). I THINK this is right, but not entirely sure, as it’s been about 40 years since I read it last. Found! It's Black Sabbat by J.B. Herman:
4. ’70s/early ’80s: the cover was purple and had the embossed, in black, wide-stretched jaws of a big cat, all fangs and stuff, alluringly emblazoned upon it. Or maybe it was the open maw of a panther of some sort, open mouth that sort of encompassed the book’s title, more a line drawing than a painting, and embossed. I could have sworn the book was about an ancient evil/demon being released and its grisly killing spree in a museum of some sort (*not* The Relic and *not* Panther!).
5. ’70s/early ’80s: had to have been a best seller, because it was in the supermarket. This was the rural south, so they weren't gonna waste shelf space. The cover was dark and depicted a sort of "Solid Gold" (the tv music show) stage set, with large squares. It was a photograph, not a painted cover. And there was a single hand extended from behind one of them. It was inexplicable, and SO eerie, and scared me silly, and I don't even know why.
6. ’89 or ’90. It's a compilation of horror stories. The cover is black on top with a square of illustration covering the bottom. It features a man in ill-fitting clothing. Ripped jeans, letterman's jacket (red?) and I know there's a severed leg somewhere in there (the memory of it is kind of fading). He's crouching in the middle of a forest. A few items are around him, maybe from his victims (a red shoe?). He looks animalistic, perhaps not to the extent of being a werewolf, but like he degraded into insanity.
The one story I read was written in the form of log entries by the captain. His crew is in space looking for a habitable planet, from what I recall. The planet they land on is full of vegetation and the only life they can see are butterflies. The planet is full of butterflies. Previously I had thought that by eating some of the vegetation that the crew, having landed there, had turned into butterflies, themselves. I don't even remember what happened to them. Eventually, the captain's log degenerated as his mind failed him. The last page was full of big loops and curly cues as he forgot how to write.
Found! It's "Gestation" by Bruce Jones: