So, I got that Man After Man book I mentioned here. It chilled me as a kid. Nowadays it looks like Deviantart's tribute to Where The Wild Things Are. After decades of comics, anime, fantasy art bad and good, I can't imagine getting freaked by monster illustrations. OTOH I loved stuff like Where the Wild Things Are when I was a kid, so I think these monster pictures freaked me not because I was vulnerable to monster pictures, but because I mistook the book's veneer of scientificness for scientific certainty. When I knew monsters were imaginary I didn't take them too seriously, or rather I didn't take them seriously in the wrong way. But when I thought they were, or might be, verified truth I let those monsters terrify me the way monsters are meant too. It's a bit like religion... stories that seem like obvious fantasy to an unreceptive perspective become deeply powerful truths from a receptive perspective.
I guess horror fiction in general works that way. H. P. Lovecraft and Blair Witch Project are more familiar cases of stuff that's either deeply frightening or Flight-of-the-Valkyries-played-by-a-kazoo-orchestra goofy, depending on whether one is receptive or not.
Oh, there's one other thing... the images I remembered from the book are nowhere to be found in the book. My visual literacy was rather poor as a child, so my recollections of the book's images wound up being a tribute to the imaginative rather than reconstructive nature of memory.