I found out the error. This knowledge might be common, but it was not to me and will prove useful to other users.
EPUB format books are zipped collections of meta data and individual HTML files. The HTML files are usually split at a little below 100 KB each. I had set the split to 1,680 KB when converting my books into EPUB through Calibre. After setting the value back down the books are readable from cover to end.
What this tells us is that the EZ Reader reads the HTML from its header until the point where its RAM can not hold anymore of the file. Splitting the files allows the previous file to be dropped from RAM and the next to fill it; producing the sensation that the book is linear at the cost of loading times.
My large EPUB, non split HTMLs, filled RAM and the book stopped at the point where the device couldn't read anymore.
It turns out that this error I ran into was not a device fault, merely a limitation.
Thank you Dale for your suggestion about comparing them with the books from this forum. That is what set my brain on the right track. I didn't even know there were public domain books on this site! Thank you Ducks for the file size tip, it was comparing the file sizes that hung the light above my head. Lastly, thank you Jenifer-with-one-n for offering support so quickly from the device makers' side.