Following up from this thread:
https://www.mobileread.com/forums/sho...d.php?t=191260
Some DRM free books downloaded from iTunes has .ibooks extension as produced by iAuthor vs normal .epub.
The suggestion to try from above thread was to rename .ibooks to .epub and open it with Calibre.
As far I have tested calibre viewer fails to open them claiming DRM protection which is not true.
Instead one can use FBreader (
http://fbreader.org) to view such books. Text and images are visible. Some functionality like embedded audio is not available.