Downloaded a bunch of books which were compressed in a .xdi file. How do I open/extract the internal data/files? I go to open, and Ubuntu does not recognize/nor is there a seemingly effective application..
XDI is a proprietary (closed-source) compressed format for ISO files (i.e disk images). You can't open/extract it directly on Ubuntu.
The format was developed and can only be created/opened/extracted with WinArchiver, and you can download a trial, 30-day version here. You may not need Windows though; try running it using Wine in Ubuntu.