[DiapDealer]

Sounds like a fixed layout ePub (possibly). I believe Apple, B&N and Kobo all have some sort of variant of this beast. Kobo's and Apple's flavor are pretty much identical... unzip the ePub and look for <option name="fixed-layout">true</option> in one of the files in the META-INF folder (most likely "com.apple.ibooks.display-options.xml")

B&N, very strangely, relies simply on a portion of the file-name to process their "Digital Replica (DRP) formatted ePubs. B&N's DRP ePubs will have a file-name that ends in "_drp.epub."

All of these files stray from the standard ePub spec in various proprietary way(s).

All of which means (if what you have is indeed one of these) that you're going to have the exact same issues with trying to convert one of these files as people do when trying to convert PDFs. Often times, it's just not worth the manual effort involved.