The earlier Mercer Mayer books were considered too scary to be widely commercial or TV-ready. I don't know if that's why Mayer switched to anthropomorphic 'funny' monsters in books aimed at even younger kids, but I think those books have been adapted for TV. Er, Little Critter and Little Monster.
We liked One Monster After Another much better, but Wikipedia tells me that actually it barely predates the 'funny' monster books. Even it really isn't scary, nor is it meant to be, but the monsters are much more fantastical, less anthropomorphic, more numinous.
Mayer also illustrated other writers' books. We liked the Great Brain books, but they were sometimes seen as encouraging rebellion and bad behavior.
We liked Charles Addams when we were kids, too, though. What we thought was scary was some mostly-illustrations book about a witch (old school, with giant brambles and, I think, a threat of cannibalism) that had creepy, surreal pictures. No idea now who did that one. It wasn't uncommon back in the 70s, but I doubt I've seen a copy of it in the last 25 years, whatever it was.
