A beholder (singular) is someone who learns (⬜), gains knowledge of (⬜), and or awareness of (⬜) something with senses (⬜), primarily through sight (⬜) more often that not. 'Beholders' is the plural, so just multiple of that (⬜). Based on the story, it's most likely the people who read the books, or the 2 dragons (or spectres, we do want to be all inclusive) who had nothing else to do/is constantly doing so everywhere in the world, so most likely to be more centered on Daniel, Charlotte*, Ruin, and or Animi/Barren**.
* - Deceased by current known information
** - Idk, maybe deceased, maybe not, probably like what's-his-face in Turtle's mind
'Eyes' being plural is probably just because number agreement rule, but it could also refer to more, mostly to Animi, as I'm pretty sure he sees all, which I imagine as lots of eyes, watching everything secretly for the most part.
The title is basically "Eyes of the Observers," which could technically refer to us, as the readers, who have to learn (✅), gain knowledge of (✅), and get awareness of (✅) things in the story, as well as the story as a whole, through our senses (✅) by reading, aka sight (✅), and there are multiple of us (✅). Every checked box is for the checklist of key parts to the definition of a beholder, listed through (⬜) at the beginning. So by definition, we are beholders of this story.
Eye of the beholder, an actual phrase, is essentially the view person or being who observes, and typically means the beholder gets to paint the view, meaning it could also be referring to our excellent and underpaid author over here, however the plural would counteract that, unless we, or, less likely if this ends up being the case, one of the 4 listed as possible 'Beholders' also is connected.
Can't wait for every bit of this to be entirely meaningless and to have no relation to the story whatsoever.