As a kid I had a book about aliens. It was a pseudo-encyclopedia of sorts with different kinds of articles and chapters about extraterrestrials. I was so young at the time, I don't remember a great deal about the text, but many of the pictures were quite striking. My dad got it for me and we loved it, though my mother was not so keen on some of the gore. As I remember, there was a disembodied eye on the cover and one image inside the book was of a hand holding several eyeballs.
Alien World: The Complete Illustrated Guide by Steven Eisler
This was an art book of sci-fi aliens illustrated by various authors, with lurid descriptions invented by Mr. Eisler.
Disembodied eye on the cover:
Hand holding several eyeballs:
The likeliest answer is "Barlowe's Guide to Extraterrestrials".
I can't vouch for the hand holding the eyeballs but maybe that was a different cover.